1245 lines
65 KiB
Plaintext
1245 lines
65 KiB
Plaintext
.. index:: variable
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variable command
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================
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Syntax
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""""""
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.. parsed-literal::
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variable name style args ...
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* name = name of variable to define
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* style = *delete* or *index* or *loop* or *world* or *universe* or
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*uloop* or *string* or *format* or *getenv* or *file* or *atomfile* or *python* or *equal* or *atom*
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.. parsed-literal::
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*delete* = no args
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*index* args = one or more strings
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*loop* args = N
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N = integer size of loop, loop from 1 to N inclusive
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*loop* args = N pad
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N = integer size of loop, loop from 1 to N inclusive
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pad = all values will be same length, e.g. 001, 002, ..., 100
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*loop* args = N1 N2
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N1,N2 = loop from N1 to N2 inclusive
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*loop* args = N1 N2 pad
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N1,N2 = loop from N1 to N2 inclusive
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pad = all values will be same length, e.g. 050, 051, ..., 100
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*world* args = one string for each partition of processors
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*universe* args = one or more strings
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*uloop* args = N
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N = integer size of loop
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*uloop* args = N pad
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N = integer size of loop
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pad = all values will be same length, e.g. 001, 002, ..., 100
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*string* arg = one string
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*format* args = vname fstr
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vname = name of equal-style variable to evaluate
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fstr = C-style format string
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*getenv* arg = one string
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*file* arg = filename
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*atomfile* arg = filename
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*python* arg = function
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*equal* or *atom* args = one formula containing numbers, thermo keywords, math operations, group functions, atom values and vectors, compute/fix/variable references
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numbers = 0.0, 100, -5.4, 2.8e-4, etc
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constants = PI
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thermo keywords = vol, ke, press, etc from :doc:`thermo_style <thermo_style>`
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math operators = (), -x, x+y, x-y, x*y, x/y, x^y, x%y,
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x == y, x != y, x < y, x <= y, x > y, x >= y, x && y, x || y, !x
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math functions = sqrt(x), exp(x), ln(x), log(x), abs(x),
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sin(x), cos(x), tan(x), asin(x), acos(x), atan(x), atan2(y,x),
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random(x,y,z), normal(x,y,z), ceil(x), floor(x), round(x)
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ramp(x,y), stagger(x,y), logfreq(x,y,z), logfreq2(x,y,z),
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stride(x,y,z), stride2(x,y,z,a,b,c),
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vdisplace(x,y), swiggle(x,y,z), cwiggle(x,y,z)
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group functions = count(group), mass(group), charge(group),
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xcm(group,dim), vcm(group,dim), fcm(group,dim),
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bound(group,dir), gyration(group), ke(group),
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angmom(group,dim), torque(group,dim),
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inertia(group,dimdim), omega(group,dim)
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region functions = count(group,region), mass(group,region), charge(group,region),
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xcm(group,dim,region), vcm(group,dim,region), fcm(group,dim,region),
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bound(group,dir,region), gyration(group,region), ke(group,reigon),
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angmom(group,dim,region), torque(group,dim,region),
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inertia(group,dimdim,region), omega(group,dim,region)
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special functions = sum(x), min(x), max(x), ave(x), trap(x), slope(x), gmask(x), rmask(x), grmask(x,y), next(x)
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atom value = id[i], mass[i], type[i], mol[i], x[i], y[i], z[i], vx[i], vy[i], vz[i], fx[i], fy[i], fz[i], q[i]
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atom vector = id, mass, type, mol, x, y, z, vx, vy, vz, fx, fy, fz, q
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compute references = c_ID, c_ID[i], c_ID[i][j]
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fix references = f_ID, f_ID[i], f_ID[i][j]
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variable references = v_name, v_name[i]
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Examples
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""""""""
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.. parsed-literal::
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variable x index run1 run2 run3 run4 run5 run6 run7 run8
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variable LoopVar loop $n
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variable beta equal temp/3.0
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variable b1 equal x[234]+0.5*vol
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variable b1 equal "x[234] + 0.5*vol"
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variable b equal xcm(mol1,x)/2.0
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variable b equal c_myTemp
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variable b atom x*y/vol
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variable foo string myfile
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variable myPy python increase
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variable f file values.txt
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variable temp world 300.0 310.0 320.0 ${Tfinal}
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variable x universe 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
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variable x uloop 15 pad
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variable str format x %.6g
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variable x delete
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Description
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"""""""""""
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This command assigns one or more strings to a variable name for
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evaluation later in the input script or during a simulation.
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Variables can thus be useful in several contexts. A variable can be
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defined and then referenced elsewhere in an input script to become
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part of a new input command. For variable styles that store multiple
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strings, the :doc:`next <next>` command can be used to increment which
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string is assigned to the variable. Variables of style *equal* store
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a formula which when evaluated produces a single numeric value which
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can be output either directly (see the :doc:`print <print>`, :doc:`fix print <fix_print>`, and :doc:`run every <run>` commands) or as part
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of thermodynamic output (see the :doc:`thermo_style <thermo_style>`
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command), or used as input to an averaging fix (see the :doc:`fix ave/time <fix_ave_time>` command). Variables of style *atom* store
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a formula which when evaluated produces one numeric value per atom
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which can be output to a dump file (see the :doc:`dump custom <dump>`
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command) or used as input to an averaging fix (see the :doc:`fix ave/spatial <fix_ave_spatial>` and :doc:`fix ave/atom <fix_ave_atom>`
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commands). Variables of style *atomfile* can be used anywhere in an
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input script that atom-style variables are used; they get their
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per-atom values from a file rather than from a formula. Variables can
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be hooked to Python functions using code you provide, so that the
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variable gets its value from the evaluation of the Python code.
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.. warning::
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As discussed in :ref:`Section 3.2 <cmd_2>`
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of the manual, an input script can use "immediate" variables, specified
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as $(formula) with parenthesis, where the formula has the same syntax
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as equal-style variables described on this page. This is a convenient
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way to evaluate a formula immediately without using the variable command
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to define a named variable and then evaluate that variable. See below
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for a more detailed discussion of this feature.
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In the discussion that follows, the "name" of the variable is the
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arbitrary string that is the 1st argument in the variable command.
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This name can only contain alphanumeric characters and underscores.
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The "string" is one or more of the subsequent arguments. The "string"
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can be simple text as in the 1st example above, it can contain other
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variables as in the 2nd example, or it can be a formula as in the 3rd
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example. The "value" is the numeric quantity resulting from
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evaluation of the string. Note that the same string can generate
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different values when it is evaluated at different times during a
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simulation.
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.. warning::
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When the input script line is encountered that defines
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a variable of style *equal* or *atom* or *python* that contains a
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formula or Python code, the formula is NOT immediately evaluated.
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It will be evaluated every time when the variable is **used** instead.
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If you simply want to evaluate a formula in place you can use as
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so-called. See the section below about "Immediate Evaluation
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of Variables" for more details on the topic. This is also true of
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a *format* style variable since it evaluates another variable when
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it is invoked.
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.. warning::
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Variables of style *equal* and *atom* can be used as
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inputs to various other LAMMPS commands which evaluate their formulas
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as needed, e.g. at different timesteps during a :doc:`run <run>`.
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Variables of style *python* can be used in place of an equal-style
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variable so long as the associated Python function, as defined by the
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:doc:`python <python>` command, returns a numeric value. Thus any
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command that states it can use an equal-style variable as an argument,
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can also use such a python-style variable. This means that when the
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LAMMPS command evaluates the variable, the Python function will be
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executed.
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.. warning::
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When a variable command is encountered in the input
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script and the variable name has already been specified, the command
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is ignored. This means variables can NOT be re-defined in an input
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script (with two exceptions, read further). This is to allow an input
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script to be processed multiple times without resetting the variables;
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see the :doc:`jump <jump>` or :doc:`include <include>` commands. It also
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means that using the :ref:`command-line switch <start_7>`
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-var will override a corresponding index variable setting in the input
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script.
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There are two exceptions to this rule. First, variables of style
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*string*, *getenv*, *equal*, *atom*, and *python* ARE redefined each
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time the command is encountered. This allows these style of variables
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to be redefined multiple times in an input script. In a loop, this
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means the formula associated with an *equal* or *atom* style variable
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can change if it contains a substitution for another variable, e.g. $x
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or v_x.
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Second, as described below, if a variable is iterated on to the end of
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its list of strings via the :doc:`next <next>` command, it is removed
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from the list of active variables, and is thus available to be
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re-defined in a subsequent variable command. The *delete* style does
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the same thing.
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----------
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:ref:`This section <cmd_2>` of the manual explains how
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occurrences of a variable name in an input script line are replaced by
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the variable's string. The variable name can be referenced as $x if
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the name "x" is a single character, or as ${LoopVar} if the name
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"LoopVar" is one or more characters.
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As described below, for variable styles *index*, *loop*, *file*,
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*universe*, and *uloop*, which string is assigned to a variable can be
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incremented via the :doc:`next <next>` command. When there are no more
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strings to assign, the variable is exhausted and a flag is set that
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causes the next :doc:`jump <jump>` command encountered in the input
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script to be skipped. This enables the construction of simple loops
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in the input script that are iterated over and then exited from.
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As explained above, an exhausted variable can be re-used in an input
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script. The *delete* style also removes the variable, the same as if
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it were exhausted, allowing it to be redefined later in the input
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script or when the input script is looped over. This can be useful
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when breaking out of a loop via the :doc:`if <if>` and :doc:`jump <jump>`
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commands before the variable would become exhausted. For example,
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.. parsed-literal::
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label loop
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variable a loop 5
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print "A = $a"
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if "$a > 2" then "jump in.script break"
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next a
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jump in.script loop
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label break
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variable a delete
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----------
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This section describes how all the various variable styles are defined
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and what they store. Except for the *equal* and *atom* styles,
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which are explaine in the next section.
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Many of the styles store one or more strings. Note that a single
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string can contain spaces (multiple words), if it is enclosed in
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quotes in the variable command. When the variable is substituted for
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in another input script command, its returned string will then be
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interpreted as multiple arguments in the expanded command.
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For the *index* style, one or more strings are specified. Initially,
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the 1st string is assigned to the variable. Each time a
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:doc:`next <next>` command is used with the variable name, the next
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string is assigned. All processors assign the same string to the
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variable.
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*Index* style variables with a single string value can also be set by
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using the command-line switch -var; see :ref:`this section <start_7>` for details.
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The *loop* style is identical to the *index* style except that the
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strings are the integers from 1 to N inclusive, if only one argument N
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is specified. This allows generation of a long list of runs
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(e.g. 1000) without having to list N strings in the input script.
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Initially, the string "1" is assigned to the variable. Each time a
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:doc:`next <next>` command is used with the variable name, the next
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string ("2", "3", etc) is assigned. All processors assign the same
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string to the variable. The *loop* style can also be specified with
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two arguments N1 and N2. In this case the loop runs from N1 to N2
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inclusive, and the string N1 is initially assigned to the variable.
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N1 <= N2 and N2 >= 0 is required.
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For the *world* style, one or more strings are specified. There must
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be one string for each processor partition or "world". See :ref:`this section <start_7>` of the manual for information on
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running LAMMPS with multiple partitions via the "-partition"
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command-line switch. This variable command assigns one string to each
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world. All processors in the world are assigned the same string. The
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next command cannot be used with *equal* style variables, since there
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is only one value per world. This style of variable is useful when
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you wish to run different simulations on different partitions, or when
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performing a parallel tempering simulation (see the
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:doc:`temper <temper>` command), to assign different temperatures to
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different partitions.
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For the *universe* style, one or more strings are specified. There
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must be at least as many strings as there are processor partitions or
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"worlds". See :ref:`this page <start_7>` for information
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on running LAMMPS with multiple partitions via the "-partition"
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command-line switch. This variable command initially assigns one
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string to each world. When a :doc:`next <next>` command is encountered
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using this variable, the first processor partition to encounter it, is
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assigned the next available string. This continues until all the
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variable strings are consumed. Thus, this command can be used to run
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50 simulations on 8 processor partitions. The simulations will be run
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one after the other on whatever partition becomes available, until
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they are all finished. *Universe* style variables are incremented
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using the files "tmp.lammps.variable" and "tmp.lammps.variable.lock"
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which you will see in your directory during such a LAMMPS run.
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The *uloop* style is identical to the *universe* style except that the
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strings are the integers from 1 to N. This allows generation of long
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list of runs (e.g. 1000) without having to list N strings in the input
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script.
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For the *string* style, a single string is assigned to the variable.
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The only difference between this and using the *index* style with a
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single string is that a variable with *string* style can be redefined.
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E.g. by another command later in the input script, or if the script is
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read again in a loop.
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For the *format* style, an equal-style variable is specified along
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with a C-style format string, e.g. "%f" or "%.10g", which must be
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appropriate for formatting a double-precision floating-point value.
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This allows an equal-style variable to be formatted specifically for
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output as a string, e.g. by the :doc:`print <print>` command, if the
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default format "%.15g" has too much precision.
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For the *getenv* style, a single string is assigned to the variable
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which should be the name of an environment variable. When the
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variable is evaluated, it returns the value of the environment
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variable, or an empty string if it not defined. This style of
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variable can be used to adapt the behavior of LAMMPS input scripts via
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environment variable settings, or to retrieve information that has
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been previously stored with the :doc:`shell putenv <shell>` command.
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Note that because environment variable settings are stored by the
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operating systems, they persist beyond a :doc:`clear <clear>` command.
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For the *file* style, a filename is provided which contains a list of
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strings to assign to the variable, one per line. The strings can be
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numeric values if desired. See the discussion of the next() function
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below for equal-style variables, which will convert the string of a
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file-style variable into a numeric value in a formula.
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When a file-style variable is defined, the file is opened and the
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string on the first line is read and stored with the variable. This
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means the variable can then be evaluated as many times as desired and
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will return that string. There are two ways to cause the next string
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from the file to be read: use the :doc:`next <next>` command or the
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next() function in an equal- or atom-style variable, as discussed
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below.
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The rules for formatting the file are as follows. A comment character
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"#" can be used anywhere on a line; text starting with the comment
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character is stripped. Blank lines are skipped. The first "word" of
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a non-blank line, delimited by white space, is the "string" assigned
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to the variable.
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For the *atomfile* style, a filename is provided which contains one or
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more sets of values, to assign on a per-atom basis to the variable.
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The format of the file is described below.
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When an atomfile-style variable is defined, the file is opened and the
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first set of per-atom values are read and stored with the variable.
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This means the variable can then be evaluated as many times as desired
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and will return those values. There are two ways to cause the next
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set of per-atom values from the file to be read: use the
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:doc:`next <next>` command or the next() function in an atom-style
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variable, as discussed below.
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The rules for formatting the file are as follows. Each time a set of
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per-atom values is read, a non-blank line is searched for in the file.
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A comment character "#" can be used anywhere on a line; text starting
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with the comment character is stripped. Blank lines are skipped. The
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first "word" of a non-blank line, delimited by white space, is read as
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the count N of per-atom lines to immediately follow. N can be be the
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total number of atoms in the system, or only a subset. The next N
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lines have the following format
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.. parsed-literal::
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ID value
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where ID is an atom ID and value is the per-atom numeric value that
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will be assigned to that atom. IDs can be listed in any order.
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.. warning::
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Every time a set of per-atom lines is read, the value
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for all atoms is first set to 0.0. Thus values for atoms whose ID
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does not appear in the set, will remain 0.0.
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For the *python* style a Python function name is provided. This needs
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to match a function name specified in a :doc:`python <python>` command
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which returns a value to this variable as defined by its *return*
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keyword. For exampe these two commands would be self-consistent:
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.. parsed-literal::
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variable foo python myMultiply
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python myMultiply return v_foo format f file funcs.py
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The two commands can appear in either order so long as both are
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specified before the Python function is invoked for the first time.
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Each time the variable is evaluated, the associated Python function is
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invoked, and the value it returns is also returned by the variable.
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Since the Python function can use other LAMMPS variables as input, or
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query interal LAMMPS quantities to perform its computation, this means
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the variable can return a different value each time it is evaluated.
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The type of value stored in the variable is determined by the *format*
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keyword of the :doc:`python <python>` command. It can be an integer
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(i), floating point (f), or string (s) value. As mentioned above, if
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it is a numeric value (integer or floating point), then the
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python-style variable can be used in place of an equal-style variable
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anywhere in an input script, e.g. as an argument to another command
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that allows for equal-style variables.
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----------
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For the *equal* and *atom* styles, a single string is specified which
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represents a formula that will be evaluated afresh each time the
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variable is used. If you want spaces in the string, enclose it in
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double quotes so the parser will treat it as a single argument. For
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*equal* style variables the formula computes a scalar quantity, which
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becomes the value of the variable whenever it is evaluated. For
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*atom* style variables the formula computes one quantity for each
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atom whenever it is evaluated.
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Note that *equal* and *atom* variables can produce different values at
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different stages of the input script or at different times during a
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run. For example, if an *equal* variable is used in a :doc:`fix print <fix_print>` command, different values could be printed each
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timestep it was invoked. If you want a variable to be evaluated
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immediately, so that the result is stored by the variable instead of
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the string, see the section below on "Immediate Evaluation of
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Variables".
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The next command cannot be used with *equal* or *atom* style
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variables, since there is only one string.
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The formula for an *equal* or *atom* variable can contain a variety
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of quantities. The syntax for each kind of quantity is simple, but
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multiple quantities can be nested and combined in various ways to
|
|
build up formulas of arbitrary complexity. For example, this is a
|
|
valid (though strange) variable formula:
|
|
|
|
.. parsed-literal::
|
|
|
|
variable x equal "pe + c_MyTemp / vol^(1/3)"
|
|
|
|
Specifically, an formula can contain numbers, thermo keywords, math
|
|
operators, math functions, group functions, region functions, atom
|
|
values, atom vectors, compute references, fix references, and
|
|
references to other variables.
|
|
|
|
+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|
|
| Number | 0.2, 100, 1.0e20, -15.4, etc |
|
|
+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|
|
| Constant | PI |
|
|
+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|
|
| Thermo keywords | vol, pe, ebond, etc |
|
|
+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|
|
| Math operators | (), -x, x+y, x-y, x*y, x/y, x^y, x%y, |
|
|
+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|
|
| Math operators | (), -x, x+y, x-y, x*y, x/y, x^y, x%y, x == y, x != y, x < y, x <= y, x > y, x >= y, x && y, x || y, !x |
|
|
+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|
|
| Math functions | sqrt(x), exp(x), ln(x), log(x), abs(x), sin(x), cos(x), tan(x), asin(x), acos(x), atan(x), atan2(y,x), random(x,y,z), normal(x,y,z), ceil(x), floor(x), round(x), ramp(x,y), stagger(x,y), logfreq(x,y,z), logfreq2(x,y,z), stride(x,y,z), stride2(x,y,z,a,b,c), vdisplace(x,y), swiggle(x,y,z), cwiggle(x,y,z) |
|
|
+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|
|
| Group functions | count(ID), mass(ID), charge(ID), xcm(ID,dim), vcm(ID,dim), fcm(ID,dim), bound(ID,dir), gyration(ID), ke(ID), angmom(ID,dim), torque(ID,dim), inertia(ID,dimdim), omega(ID,dim) |
|
|
+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|
|
| Region functions | count(ID,IDR), mass(ID,IDR), charge(ID,IDR), xcm(ID,dim,IDR), vcm(ID,dim,IDR), fcm(ID,dim,IDR), bound(ID,dir,IDR), gyration(ID,IDR), ke(ID,IDR), angmom(ID,dim,IDR), torque(ID,dim,IDR), inertia(ID,dimdim,IDR), omega(ID,dim,IDR) |
|
|
+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|
|
| Special functions | sum(x), min(x), max(x), ave(x), trap(x), slope(x), gmask(x), rmask(x), grmask(x,y), next(x) |
|
|
+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|
|
| Atom values | id[i], mass[i], type[i], mol[i], x[i], y[i], z[i], vx[i], vy[i], vz[i], fx[i], fy[i], fz[i], q[i] |
|
|
+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|
|
| Atom vectors | id, mass, type, mol, x, y, z, vx, vy, vz, fx, fy, fz, q |
|
|
+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|
|
| Compute references | c_ID, c_ID[i], c_ID[i][j] |
|
|
+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|
|
| Fix references | f_ID, f_ID[i], f_ID[i][j] |
|
|
+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|
|
| Other variables | v_name, v_name[i] |
|
|
+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|
|
|
|
|
|
----------
|
|
|
|
|
|
Most of the formula elements produce a scalar value. A few produce a
|
|
per-atom vector of values. These are the atom vectors, compute
|
|
references that represent a per-atom vector, fix references that
|
|
represent a per-atom vector, and variables that are atom-style
|
|
variables. Math functions that operate on scalar values produce a
|
|
scalar value; math function that operate on per-atom vectors do so
|
|
element-by-element and produce a per-atom vector.
|
|
|
|
A formula for equal-style variables cannot use any formula element
|
|
that produces a per-atom vector. A formula for an atom-style variable
|
|
can use formula elements that produce either a scalar value or a
|
|
per-atom vector. Atom-style variables are evaluated by other commands
|
|
that define a :doc:`group <group>` on which they operate, e.g. a
|
|
:doc:`dump <dump>` or :doc:`compute <compute>` or :doc:`fix <fix>` command.
|
|
When they invoke the atom-style variable, only atoms in the group are
|
|
inlcuded in the formula evaluation. The variable evaluates to 0.0 for
|
|
atoms not in the group.
|
|
|
|
The thermo keywords allowed in a formula are those defined by the
|
|
:doc:`thermo_style custom <thermo_style>` command. Thermo keywords that
|
|
require a :doc:`compute <compute>` to calculate their values such as
|
|
"temp" or "press", use computes stored and invoked by the
|
|
:doc:`thermo_style <thermo_style>` command. This means that you can
|
|
only use those keywords in a variable if the style you are using with
|
|
the thermo_style command (and the thermo keywords associated with that
|
|
style) also define and use the needed compute. Note that some thermo
|
|
keywords use a compute indirectly to calculate their value (e.g. the
|
|
enthalpy keyword uses temp, pe, and pressure). If a variable is
|
|
evaluated directly in an input script (not during a run), then the
|
|
values accessed by the thermo keyword must be current. See the
|
|
discussion below about "Variable Accuracy".
|
|
|
|
|
|
----------
|
|
|
|
|
|
Math Operators
|
|
--------------
|
|
|
|
Math operators are written in the usual way, where the "x" and "y" in
|
|
the examples can themselves be arbitrarily complex formulas, as in the
|
|
examples above. In this syntax, "x" and "y" can be scalar values or
|
|
per-atom vectors. For example, "ke/natoms" is the division of two
|
|
scalars, where "vy+vz" is the element-by-element sum of two per-atom
|
|
vectors of y and z velocities.
|
|
|
|
Operators are evaluated left to right and have the usual C-style
|
|
precedence: unary minus and unary logical NOT operator "!" have the
|
|
highest precedence, exponentiation "^" is next; multiplication and
|
|
division and the modulo operator "%" are next; addition and
|
|
subtraction are next; the 4 relational operators "<", "<=", ">", and
|
|
">=" are next; the two remaining relational operators "==" and "!="
|
|
are next; then the logical AND operator "&&"; and finally the logical
|
|
OR operator "||" has the lowest precedence. Parenthesis can be used
|
|
to group one or more portions of a formula and/or enforce a different
|
|
order of evaluation than what would occur with the default precedence.
|
|
|
|
.. warning::
|
|
|
|
Because a unary minus is higher precedence than
|
|
exponentiation, the formula "-2^2" will evaluate to 4, not -4. This
|
|
convention is compatible with some programming languages, but not
|
|
others. As mentioned, this behavior can be easily overridden with
|
|
parenthesis; the formula "-(2^2)" will evaluate to -4.
|
|
|
|
The 6 relational operators return either a 1.0 or 0.0 depending on
|
|
whether the relationship between x and y is TRUE or FALSE. For
|
|
example the expression x<10.0 in an atom-style variable formula will
|
|
return 1.0 for all atoms whose x-coordinate is less than 10.0, and 0.0
|
|
for the others. The logical AND operator will return 1.0 if both its
|
|
arguments are non-zero, else it returns 0.0. The logical OR operator
|
|
will return 1.0 if either of its arguments is non-zero, else it
|
|
returns 0.0. The logical NOT operator returns 1.0 if its argument is
|
|
0.0, else it returns 0.0.
|
|
|
|
These relational and logical operators can be used as a masking or
|
|
selection operation in a formula. For example, the number of atoms
|
|
whose properties satifsy one or more criteria could be calculated by
|
|
taking the returned per-atom vector of ones and zeroes and passing it
|
|
to the :doc:`compute reduce <compute_reduce>` command.
|
|
|
|
|
|
----------
|
|
|
|
|
|
Math Functions
|
|
--------------
|
|
|
|
Math functions are specified as keywords followed by one or more
|
|
parenthesized arguments "x", "y", "z", each of which can themselves be
|
|
arbitrarily complex formulas. In this syntax, the arguments can
|
|
represent scalar values or per-atom vectors. In the latter case, the
|
|
math operation is performed on each element of the vector. For
|
|
example, "sqrt(natoms)" is the sqrt() of a scalar, where "sqrt(y*z)"
|
|
yields a per-atom vector with each element being the sqrt() of the
|
|
product of one atom's y and z coordinates.
|
|
|
|
Most of the math functions perform obvious operations. The ln() is
|
|
the natural log; log() is the base 10 log.
|
|
|
|
The random(x,y,z) function takes 3 arguments: x = lo, y = hi, and z =
|
|
seed. It generates a uniform random number between lo and hi. The
|
|
normal(x,y,z) function also takes 3 arguments: x = mu, y = sigma, and
|
|
z = seed. It generates a Gaussian variate centered on mu with
|
|
variance sigma^2. In both cases the seed is used the first time the
|
|
internal random number generator is invoked, to initialize it. For
|
|
equal-style variables, every processor uses the same seed so that they
|
|
each generate the same sequence of random numbers. For atom-style
|
|
variables, a unique seed is created for each processor, based on the
|
|
specified seed. This effectively generates a different random number
|
|
for each atom being looped over in the atom-style variable.
|
|
|
|
.. warning::
|
|
|
|
Internally, there is just one random number generator
|
|
for all equal-style variables and one for all atom-style variables.
|
|
If you define multiple variables (of each style) which use the
|
|
random() or normal() math functions, then the internal random number
|
|
generators will only be initialized once, which means only one of the
|
|
specified seeds will determine the sequence of generated random
|
|
numbers.
|
|
|
|
The ceil(), floor(), and round() functions are those in the C math
|
|
library. Ceil() is the smallest integer not less than its argument.
|
|
Floor() if the largest integer not greater than its argument. Round()
|
|
is the nearest integer to its argument.
|
|
|
|
The ramp(x,y) function uses the current timestep to generate a value
|
|
linearly intepolated between the specified x,y values over the course
|
|
of a run, according to this formula:
|
|
|
|
.. parsed-literal::
|
|
|
|
value = x + (y-x) * (timestep-startstep) / (stopstep-startstep)
|
|
|
|
The run begins on startstep and ends on stopstep. Startstep and
|
|
stopstep can span multiple runs, using the *start* and *stop* keywords
|
|
of the :doc:`run <run>` command. See the :doc:`run <run>` command for
|
|
details of how to do this.
|
|
|
|
The stagger(x,y) function uses the current timestep to generate a new
|
|
timestep. X,y > 0 and x > y are required. The generated timesteps
|
|
increase in a staggered fashion, as the sequence
|
|
x,x+y,2x,2x+y,3x,3x+y,etc. For any current timestep, the next
|
|
timestep in the sequence is returned. Thus if stagger(1000,100) is
|
|
used in a variable by the :doc:`dump_modify every <dump_modify>`
|
|
command, it will generate the sequence of output timesteps:
|
|
|
|
.. parsed-literal::
|
|
|
|
100,1000,1100,2000,2100,3000,etc
|
|
|
|
The logfreq(x,y,z) function uses the current timestep to generate a
|
|
new timestep. X,y,z > 0 and y < z are required. The generated
|
|
timesteps are on a base-z logarithmic scale, starting with x, and the
|
|
y value is how many of the z-1 possible timesteps within one
|
|
logarithmic interval are generated. I.e. the timesteps follow the
|
|
sequence x,2x,3x,...y*x,x*z,2x*z,3x*z,...y*x*z,x*z^2,2x*z^2,etc. For
|
|
any current timestep, the next timestep in the sequence is returned.
|
|
Thus if logfreq(100,4,10) is used in a variable by the :doc:`dump_modify every <dump_modify>` command, it will generate this sequence of
|
|
output timesteps:
|
|
|
|
.. parsed-literal::
|
|
|
|
100,200,300,400,1000,2000,3000,4000,10000,20000,etc
|
|
|
|
The logfreq2(x,y,z) function is similar to logfreq, except a single
|
|
logarithmic interval is divided into y equally-spaced timesteps and
|
|
all of them are output. Y < z is not required. Thus, if
|
|
logfreq2(100,18,10) is used in a variable by the :doc:`dump_modify every <dump_modify>` command, then the interval between 100 and
|
|
1000 is divided as 900/18 = 50 steps, and it will generate the
|
|
sequence of output timesteps:
|
|
|
|
.. parsed-literal::
|
|
|
|
100,150,200,...950,1000,1500,2000,...9500,10000,15000,etc
|
|
|
|
The stride(x,y,z) function uses the current timestep to generate a new
|
|
timestep. X,y >= 0 and z > 0 and x <= y are required. The generated
|
|
timesteps increase in increments of z, from x to y, i.e. it generates
|
|
the sequece x,x+z,x+2z,...,y. If y-x is not a multiple of z, then
|
|
similar to the way a for loop operates, the last value will be one
|
|
that does not exceed y. For any current timestep, the next timestep
|
|
in the sequence is returned. Thus if stride(1000,2000,100) is used
|
|
in a variable by the :doc:`dump_modify every <dump_modify>` command, it
|
|
will generate the sequence of output timesteps:
|
|
|
|
.. parsed-literal::
|
|
|
|
1000,1100,1200, ... ,1900,2000
|
|
|
|
The stride2(x,y,z,a,b,c) function is similar to the stride() function
|
|
except it generates two sets of strided timesteps, one at a coarser
|
|
level and one at a finer level. Thus it is useful for debugging,
|
|
e.g. to produce output every timestep at the point in simulation when
|
|
a problem occurs. X,y >= 0 and z > 0 and x <= y are required, as are
|
|
a,b >= 0 and c > 0 and a < b. Also, a >= x and b <= y are required so
|
|
that the second stride is inside the first. The generated timesteps
|
|
increase in increments of z, starting at x, until a is reached. At
|
|
that point the timestep increases in increments of c, from a to b,
|
|
then after b, increments by z are resumed until y is reached. For any
|
|
current timestep, the next timestep in the sequence is returned. Thus
|
|
if stride(1000,2000,100,1350,1360,1) is used in a variable by the
|
|
:doc:`dump_modify every <dump_modify>` command, it will generate the
|
|
sequence of output timesteps:
|
|
|
|
.. parsed-literal::
|
|
|
|
1000,1100,1200,1300,1350,1351,1352, ... 1359,1360,1400,1500, ... ,2000
|
|
|
|
The vdisplace(x,y) function takes 2 arguments: x = value0 and y =
|
|
velocity, and uses the elapsed time to change the value by a linear
|
|
displacement due to the applied velocity over the course of a run,
|
|
according to this formula:
|
|
|
|
.. parsed-literal::
|
|
|
|
value = value0 + velocity*(timestep-startstep)*dt
|
|
|
|
where dt = the timestep size.
|
|
|
|
The run begins on startstep. Startstep can span multiple runs, using
|
|
the *start* keyword of the :doc:`run <run>` command. See the
|
|
:doc:`run <run>` command for details of how to do this. Note that the
|
|
:doc:`thermo_style <thermo_style>` keyword elaplong =
|
|
timestep-startstep.
|
|
|
|
The swiggle(x,y,z) and cwiggle(x,y,z) functions each take 3 arguments:
|
|
x = value0, y = amplitude, z = period. They use the elapsed time to
|
|
oscillate the value by a sin() or cos() function over the course of a
|
|
run, according to one of these formulas, where omega = 2 PI / period:
|
|
|
|
.. parsed-literal::
|
|
|
|
value = value0 + Amplitude * sin(omega*(timestep-startstep)*dt)
|
|
value = value0 + Amplitude * (1 - cos(omega*(timestep-startstep)*dt))
|
|
|
|
where dt = the timestep size.
|
|
|
|
The run begins on startstep. Startstep can span multiple runs, using
|
|
the *start* keyword of the :doc:`run <run>` command. See the
|
|
:doc:`run <run>` command for details of how to do this. Note that the
|
|
:doc:`thermo_style <thermo_style>` keyword elaplong =
|
|
timestep-startstep.
|
|
|
|
|
|
----------
|
|
|
|
|
|
Group and Region Functions
|
|
--------------------------
|
|
|
|
Group functions are specified as keywords followed by one or two
|
|
parenthesized arguments. The first argument *ID* is the group-ID.
|
|
The *dim* argument, if it exists, is *x* or *y* or *z*. The *dir*
|
|
argument, if it exists, is *xmin*, *xmax*, *ymin*, *ymax*, *zmin*, or
|
|
*zmax*. The *dimdim* argument, if it exists, is *xx* or *yy* or *zz*
|
|
or *xy* or *yz* or *xz*.
|
|
|
|
The group function count() is the number of atoms in the group. The
|
|
group functions mass() and charge() are the total mass and charge of
|
|
the group. Xcm() and vcm() return components of the position and
|
|
velocity of the center of mass of the group. Fcm() returns a
|
|
component of the total force on the group of atoms. Bound() returns
|
|
the min/max of a particular coordinate for all atoms in the group.
|
|
Gyration() computes the radius-of-gyration of the group of atoms. See
|
|
the :doc:`compute gyration <compute_gyration>` command for a definition
|
|
of the formula. Angmom() returns components of the angular momentum
|
|
of the group of atoms around its center of mass. Torque() returns
|
|
components of the torque on the group of atoms around its center of
|
|
mass, based on current forces on the atoms. Inertia() returns one of
|
|
6 components of the symmetric inertia tensor of the group of atoms
|
|
around its center of mass, ordered as Ixx,Iyy,Izz,Ixy,Iyz,Ixz.
|
|
Omega() returns components of the angular velocity of the group of
|
|
atoms around its center of mass.
|
|
|
|
Region functions are specified exactly the same way as group functions
|
|
except they take an extra final argument *IDR* which is the region ID.
|
|
The function is computed for all atoms that are in both the group and
|
|
the region. If the group is "all", then the only criteria for atom
|
|
inclusion is that it be in the region.
|
|
|
|
|
|
----------
|
|
|
|
|
|
Special Functions
|
|
-----------------
|
|
|
|
Special functions take specific kinds of arguments, meaning their
|
|
arguments cannot be formulas themselves.
|
|
|
|
The sum(x), min(x), max(x), ave(x), trap(x), and slope(x) functions
|
|
each take 1 argument which is of the form "c_ID" or "c_ID[N]" or
|
|
"f_ID" or "f_ID[N]". The first two are computes and the second two
|
|
are fixes; the ID in the reference should be replaced by the ID of a
|
|
compute or fix defined elsewhere in the input script. The compute or
|
|
fix must produce either a global vector or array. If it produces a
|
|
global vector, then the notation without "[N]" should be used. If
|
|
it produces a global array, then the notation with "[N]" should be
|
|
used, when N is an integer, to specify which column of the global
|
|
array is being referenced.
|
|
|
|
These functions operate on the global vector of inputs and reduce it
|
|
to a single scalar value. This is analagous to the operation of the
|
|
:doc:`compute reduce <compute_reduce>` command, which invokes the same
|
|
functions on per-atom and local vectors.
|
|
|
|
The sum() function calculates the sum of all the vector elements. The
|
|
min() and max() functions find the minimum and maximum element
|
|
respectively. The ave() function is the same as sum() except that it
|
|
divides the result by the length of the vector.
|
|
|
|
The trap() function is the same as sum() except the first and last
|
|
elements are multiplied by a weighting factor of 1/2 when performing
|
|
the sum. This effectively implements an integration via the
|
|
trapezoidal rule on the global vector of data. I.e. consider a set of
|
|
points, equally spaced by 1 in their x coordinate: (1,V1), (2,V2),
|
|
..., (N,VN), where the Vi are the values in the global vector of
|
|
length N. The integral from 1 to N of these points is trap(). When
|
|
appropriately normalized by the timestep size, this function is useful
|
|
for calculating integrals of time-series data, like that generated by
|
|
the :doc:`fix ave/correlate <fix_ave_correlate>` command.
|
|
|
|
The slope() function uses linear regression to fit a line to the set
|
|
of points, equally spaced by 1 in their x coordinate: (1,V1), (2,V2),
|
|
..., (N,VN), where the Vi are the values in the global vector of
|
|
length N. The returned value is the slope of the line. If the line
|
|
has a single point or is vertical, it returns 1.0e20.
|
|
|
|
The gmask(x) function takes 1 argument which is a group ID. It
|
|
can only be used in atom-style variables. It returns a 1 for
|
|
atoms that are in the group, and a 0 for atoms that are not.
|
|
|
|
The rmask(x) function takes 1 argument which is a region ID. It can
|
|
only be used in atom-style variables. It returns a 1 for atoms that
|
|
are in the geometric region, and a 0 for atoms that are not.
|
|
|
|
The grmask(x,y) function takes 2 arguments. The first is a group ID,
|
|
and the second is a region ID. It can only be used in atom-style
|
|
variables. It returns a 1 for atoms that are in both the group and
|
|
region, and a 0 for atoms that are not in both.
|
|
|
|
The next(x) function takes 1 argument which is a variable ID (not
|
|
"v_foo", just "foo"). It must be for a file-style or atomfile-style
|
|
variable. Each time the next() function is invoked (i.e. each time
|
|
the equal-style or atom-style variable is evaluated), the following
|
|
steps occur.
|
|
|
|
For file-style variables, the current string value stored by the
|
|
file-style variable is converted to a numeric value and returned by
|
|
the function. And the next string value in the file is read and
|
|
stored. Note that if the line previously read from the file was not a
|
|
numeric string, then it will typically evaluate to 0.0, which is
|
|
likely not what you want.
|
|
|
|
For atomfile-style variables, the current per-atom values stored by
|
|
the atomfile-style variable are returned by the function. And the
|
|
next set of per-atom values in the file is read and stored.
|
|
|
|
Since file-style and atomfile-style variables read and store the first
|
|
line of the file or first set of per-atoms values when they are
|
|
defined in the input script, these are the value(s) that will be
|
|
returned the first time the next() function is invoked. If next() is
|
|
invoked more times than there are lines or sets of lines in the file,
|
|
the variable is deleted, similar to how the :doc:`next <next>` command
|
|
operates.
|
|
|
|
|
|
----------
|
|
|
|
|
|
Atom Values and Vectors
|
|
-----------------------
|
|
|
|
Atom values take an integer argument I from 1 to N, where I is the
|
|
atom-ID, e.g. x[243], which means use the x coordinate of the atom
|
|
with ID = 243. Or they can take a variable name, specified as v_name,
|
|
where name is the name of the variable, like x[v_myIndex]. The
|
|
variable can be of any style except atom or atom-file variables. The
|
|
variable is evaluated and the result is expected to be numeric and is
|
|
cast to an integer (i.e. 3.4 becomes 3), to use an an index, which
|
|
must be a value from 1 to N. Note that a "formula" cannot be used as
|
|
the argument between the brackets, e.g. x[243+10] or
|
|
x[v_myIndex+1] are not allowed. To do this a single variable can be
|
|
defined that contains the needed formula.
|
|
|
|
Note that the 0 < atom-ID <= N, where N is the largest atom ID
|
|
in the system. If an ID is specified for an atom that does not
|
|
currently exist, then the generated value is 0.0.
|
|
|
|
Atom vectors generate one value per atom, so that a reference like
|
|
"vx" means the x-component of each atom's velocity will be used when
|
|
evaluating the variable.
|
|
|
|
The meaning of the different atom values and vectors is mostly
|
|
self-explanatory. Mol refers to the molecule ID of an atom, and is
|
|
only defined if an :doc:`atom_style <atom_style>` is being used that
|
|
defines molecule IDs.
|
|
|
|
Note that many other atom attributes can be used as inputs to a
|
|
variable by using the :doc:`compute property/atom <compute_property_atom>` command and then specifying
|
|
a quantity from that compute.
|
|
|
|
|
|
----------
|
|
|
|
|
|
Compute References
|
|
------------------
|
|
|
|
Compute references access quantities calculated by a
|
|
:doc:`compute <compute>`. The ID in the reference should be replaced by
|
|
the ID of a compute defined elsewhere in the input script. As
|
|
discussed in the doc page for the :doc:`compute <compute>` command,
|
|
computes can produce global, per-atom, or local values. Only global
|
|
and per-atom values can be used in a variable. Computes can also
|
|
produce a scalar, vector, or array. An equal-style variable can only
|
|
use scalar values, which means a global scalar, or an element of a
|
|
global or per-atom vector or array. Atom-style variables can use the
|
|
same scalar values. They can also use per-atom vector values. A
|
|
vector value can be a per-atom vector itself, or a column of an
|
|
per-atom array. See the doc pages for individual computes to see what
|
|
kind of values they produce.
|
|
|
|
Examples of different kinds of compute references are as follows.
|
|
There is no ambiguity as to what a reference means, since computes
|
|
only produce global or per-atom quantities, never both.
|
|
|
|
+------------+-------------------------------------------------------------------------------------------------------+
|
|
| c_ID | global scalar, or per-atom vector |
|
|
+------------+-------------------------------------------------------------------------------------------------------+
|
|
| c_ID[I] | Ith element of global vector, or atom I's value in per-atom vector, or Ith column from per-atom array |
|
|
+------------+-------------------------------------------------------------------------------------------------------+
|
|
| c_ID[I][J] | I,J element of global array, or atom I's Jth value in per-atom array |
|
|
+------------+-------------------------------------------------------------------------------------------------------+
|
|
|
|
For I and J, integers can be specified or a variable name, specified
|
|
as v_name, where name is the name of the variable. The rules for this
|
|
syntax are the same as for the "Atom Values and Vectors" discussion
|
|
above.
|
|
|
|
If a variable containing a compute is evaluated directly in an input
|
|
script (not during a run), then the values accessed by the compute
|
|
must be current. See the discussion below about "Variable Accuracy".
|
|
|
|
|
|
----------
|
|
|
|
|
|
Fix References
|
|
--------------
|
|
|
|
Fix references access quantities calculated by a :doc:`fix <compute>`.
|
|
The ID in the reference should be replaced by the ID of a fix defined
|
|
elsewhere in the input script. As discussed in the doc page for the
|
|
:doc:`fix <fix>` command, fixes can produce global, per-atom, or local
|
|
values. Only global and per-atom values can be used in a variable.
|
|
Fixes can also produce a scalar, vector, or array. An equal-style
|
|
variable can only use scalar values, which means a global scalar, or
|
|
an element of a global or per-atom vector or array. Atom-style
|
|
variables can use the same scalar values. They can also use per-atom
|
|
vector values. A vector value can be a per-atom vector itself, or a
|
|
column of an per-atom array. See the doc pages for individual fixes
|
|
to see what kind of values they produce.
|
|
|
|
The different kinds of fix references are exactly the same as the
|
|
compute references listed in the above table, where "c_" is replaced
|
|
by "f_". Again, there is no ambiguity as to what a reference means,
|
|
since fixes only produce global or per-atom quantities, never both.
|
|
|
|
+------------+-------------------------------------------------------------------------------------------------------+
|
|
| f_ID | global scalar, or per-atom vector |
|
|
+------------+-------------------------------------------------------------------------------------------------------+
|
|
| f_ID[I] | Ith element of global vector, or atom I's value in per-atom vector, or Ith column from per-atom array |
|
|
+------------+-------------------------------------------------------------------------------------------------------+
|
|
| f_ID[I][J] | I,J element of global array, or atom I's Jth value in per-atom array |
|
|
+------------+-------------------------------------------------------------------------------------------------------+
|
|
|
|
For I and J, integers can be specified or a variable name, specified
|
|
as v_name, where name is the name of the variable. The rules for this
|
|
syntax are the same as for the "Atom Values and Vectors" discussion
|
|
above.
|
|
|
|
If a variable containing a fix is evaluated directly in an input
|
|
script (not during a run), then the values accessed by the fix should
|
|
be current. See the discussion below about "Variable Accuracy".
|
|
|
|
Note that some fixes only generate quantities on certain timesteps.
|
|
If a variable attempts to access the fix on non-allowed timesteps, an
|
|
error is generated. For example, the :doc:`fix ave/time <fix_ave_time>`
|
|
command may only generate averaged quantities every 100 steps. See
|
|
the doc pages for individual fix commands for details.
|
|
|
|
|
|
----------
|
|
|
|
|
|
Variable References
|
|
-------------------
|
|
|
|
Variable references access quantities stored or calculated by other
|
|
variables, which will cause those variables to be evaluated. The name
|
|
in the reference should be replaced by the name of a variable defined
|
|
elsewhere in the input script.
|
|
|
|
As discussed on this doc page, equal-style variables generate a global
|
|
scalar numeric value; atom-style and atomfile-style variables generate
|
|
a per-atom vector of numeric values; all other variables store a
|
|
string. The formula for an equal-style variable can use any style of
|
|
variable except an atom-style or atomfile-style (unless only a single
|
|
value from the variable is accessed via a subscript). If a
|
|
string-storing variable is used, the string is converted to a numeric
|
|
value. Note that this will typically produce a 0.0 if the string is
|
|
not a numeric string, which is likely not what you want. The formula
|
|
for an atom-style variable can use any style of variable, including
|
|
other atom-style or atomfile-style variables.
|
|
|
|
Examples of different kinds of variable references are as follows.
|
|
There is no ambiguity as to what a reference means, since variables
|
|
produce only a global scalar or a per-atom vector, never both.
|
|
|
|
+-----------+-----------------------------------+
|
|
| v_name | scalar, or per-atom vector |
|
|
+-----------+-----------------------------------+
|
|
| v_name[I] | atom I's value in per-atom vector |
|
|
+-----------+-----------------------------------+
|
|
|
|
For I, an integer can be specified or a variable name, specified as
|
|
v_name, where name is the name of the variable. The rules for this
|
|
syntax are the same as for the "Atom Values and Vectors" discussion
|
|
above.
|
|
|
|
|
|
----------
|
|
|
|
|
|
**Immediate Evaluation of Variables:**
|
|
|
|
If you want an equal-style variable to be evaluated immediately, it
|
|
may be the case that you do not need to define a variable at all. See
|
|
:ref:`Section 3.2 <cmd_2>` of the manual, which
|
|
describes the use of "immediate" variables in an input script,
|
|
specified as $(formula) with parenthesis, where the formula has the
|
|
same syntax as equal-style variables described on this page. This
|
|
effectively evaluates a formula immediately without using the variable
|
|
command to define a named variable.
|
|
|
|
More generally, there is a difference between referencing a variable
|
|
with a leading $ sign (e.g. $x or ${abc}) versus with a leading "v_"
|
|
(e.g. v_x or v_abc). The former can be used in any input script
|
|
command, including a variable command. The input script parser
|
|
evaluates the reference variable immediately and substitutes its value
|
|
into the command. As explained in :ref:`Section commands 3.2 <3_2>` for "Parsing rules", you can also use
|
|
un-named "immediate" variables for this purpose. For example, a
|
|
string like this $((xlo+xhi)/2+sqrt(v_area)) in an input script
|
|
command evaluates the string between the parenthesis as an equal-style
|
|
variable formula.
|
|
|
|
Referencing a variable with a leading "v_" is an optional or required
|
|
kind of argument for some commands (e.g. the :doc:`fix ave/spatial <fix_ave_spatial>` or :doc:`dump custom <dump>` or
|
|
:doc:`thermo_style <thermo_style>` commands) if you wish it to evaluate
|
|
a variable periodically during a run. It can also be used in a
|
|
variable formula if you wish to reference a second variable. The
|
|
second variable will be evaluated whenever the first variable is
|
|
evaluated.
|
|
|
|
As an example, suppose you use this command in your input script to
|
|
define the variable "v" as
|
|
|
|
.. parsed-literal::
|
|
|
|
variable v equal vol
|
|
|
|
before a run where the simulation box size changes. You might think
|
|
this will assign the initial volume to the variable "v". That is not
|
|
the case. Rather it assigns a formula which evaluates the volume
|
|
(using the thermo_style keyword "vol") to the variable "v". If you
|
|
use the variable "v" in some other command like :doc:`fix ave/time <fix_ave_time>` then the current volume of the box will be
|
|
evaluated continuously during the run.
|
|
|
|
If you want to store the initial volume of the system, you can do it
|
|
this way:
|
|
|
|
.. parsed-literal::
|
|
|
|
variable v equal vol
|
|
variable v0 equal $v
|
|
|
|
The second command will force "v" to be evaluated (yielding the
|
|
initial volume) and assign that value to the variable "v0". Thus the
|
|
command
|
|
|
|
.. parsed-literal::
|
|
|
|
thermo_style custom step v_v v_v0
|
|
|
|
would print out both the current and initial volume periodically
|
|
during the run.
|
|
|
|
Note that it is a mistake to enclose a variable formula in double
|
|
quotes if it contains variables preceeded by $ signs. For example,
|
|
|
|
.. parsed-literal::
|
|
|
|
variable vratio equal "${vfinal}/${v0}"
|
|
|
|
This is because the quotes prevent variable substitution (see :ref:`this section <cmd_2>` on parsing input script
|
|
commands), and thus an error will occur when the formula for "vratio"
|
|
is evaluated later.
|
|
|
|
|
|
----------
|
|
|
|
|
|
**Variable Accuracy:**
|
|
|
|
Obviously, LAMMPS attempts to evaluate variables containing formulas
|
|
(*equal* and *atom* style variables) accurately whenever the
|
|
evaluation is performed. Depending on what is included in the
|
|
formula, this may require invoking a :doc:`compute <compute>`, either
|
|
directly or indirectly via a thermo keyword, or accessing a value
|
|
previously calculated by a compute, or accessing a value calculated
|
|
and stored by a :doc:`fix <fix>`. If the compute is one that calculates
|
|
the pressure or energy of the system, then these quantities need to be
|
|
tallied during the evaluation of the interatomic potentials (pair,
|
|
bond, etc) on timesteps that the variable will need the values.
|
|
|
|
LAMMPS keeps track of all of this during a :doc:`run <run>` or :doc:`energy minimization <minimize>`. An error will be generated if you
|
|
attempt to evaluate a variable on timesteps when it cannot produce
|
|
accurate values. For example, if a :doc:`thermo_style custom <thermo_style>` command prints a variable which accesses
|
|
values stored by a :doc:`fix ave/time <fix_ave_time>` command and the
|
|
timesteps on which thermo output is generated are not multiples of the
|
|
averaging frequency used in the fix command, then an error will occur.
|
|
|
|
An input script can also request variables be evaluated before or
|
|
after or in between runs, e.g. by including them in a
|
|
:doc:`print <print>` command. In this case, if a compute is needed to
|
|
evaluate a variable (either directly or indirectly), LAMMPS will not
|
|
invoke the compute, but it will use a value previously calculated by
|
|
the compute, and can do this only if it was invoked on the current
|
|
timestep. Fixes will always provide a quantity needed by a variable,
|
|
but the quantity may or may not be current. This leads to one of
|
|
three kinds of behavior:
|
|
|
|
(1) The variable may be evaluated accurately. If it contains
|
|
references to a compute or fix, and these values were calculated on
|
|
the last timestep of a preceeding run, then they will be accessed and
|
|
used by the variable and the result will be accurate.
|
|
|
|
(2) LAMMPS may not be able to evaluate the variable and will generate
|
|
an error message stating so. For example, if the variable requires a
|
|
quantity from a :doc:`compute <compute>` that has not been invoked on
|
|
the current timestep, LAMMPS will generate an error. This means, for
|
|
example, that such a variable cannot be evaluated before the first run
|
|
has occurred. Likewise, in between runs, a variable containing a
|
|
compute cannot be evaluated unless the compute was invoked on the last
|
|
timestep of the preceding run, e.g. by thermodynamic output.
|
|
|
|
One way to get around this problem is to perform a 0-timestep run
|
|
before using the variable. For example, these commands
|
|
|
|
.. parsed-literal::
|
|
|
|
variable t equal temp
|
|
print "Initial temperature = $t"
|
|
run 1000
|
|
|
|
will generate an error if the run is the first run specified in the
|
|
input script, because generating a value for the "t" variable requires
|
|
a compute for calculating the temperature to be invoked.
|
|
|
|
However, this sequence of commands would be fine:
|
|
|
|
.. parsed-literal::
|
|
|
|
run 0
|
|
variable t equal temp
|
|
print "Initial temperature = $t"
|
|
run 1000
|
|
|
|
The 0-timestep run initializes and invokes various computes, including
|
|
the one for temperature, so that the value it stores is current and
|
|
can be accessed by the variable "t" after the run has completed. Note
|
|
that a 0-timestep run does not alter the state of the system, so it
|
|
does not change the input state for the 1000-timestep run that
|
|
follows. Also note that the 0-timestep run must actually use and
|
|
invoke the compute in question (e.g. via :doc:`thermo <thermo_style>` or
|
|
:doc:`dump <dump>` output) in order for it to enable the compute to be
|
|
used in a variable after the run. Thus if you are trying to print a
|
|
variable that uses a compute you have defined, you can insure it is
|
|
invoked on the last timestep of the preceding run by including it in
|
|
thermodynamic output.
|
|
|
|
Unlike computes, :doc:`fixes <fix>` will never generate an error if
|
|
their values are accessed by a variable in between runs. They always
|
|
return some value to the variable. However, the value may not be what
|
|
you expect if the fix has not yet calculated the quantity of interest
|
|
or it is not current. For example, the :doc:`fix indent <fix_indent>`
|
|
command stores the force on the indenter. But this is not computed
|
|
until a run is performed. Thus if a variable attempts to print this
|
|
value before the first run, zeroes will be output. Again, performing
|
|
a 0-timestep run before printing the variable has the desired effect.
|
|
|
|
(3) The variable may be evaluated incorrectly and LAMMPS may have no
|
|
way to detect this has occurred. Consider the following sequence of
|
|
commands:
|
|
|
|
.. parsed-literal::
|
|
|
|
pair_coeff 1 1 1.0 1.0
|
|
run 1000
|
|
pair_coeff 1 1 1.5 1.0
|
|
variable e equal pe
|
|
print "Final potential energy = $e"
|
|
|
|
The first run is performed using one setting for the pairwise
|
|
potential defined by the :doc:`pair_style <pair_style>` and
|
|
:doc:`pair_coeff <pair_coeff>` commands. The potential energy is
|
|
evaluated on the final timestep and stored by the :doc:`compute pe <compute_pe>` compute (this is done by the
|
|
:doc:`thermo_style <thermo_style>` command). Then a pair coefficient is
|
|
changed, altering the potential energy of the system. When the
|
|
potential energy is printed via the "e" variable, LAMMPS will use the
|
|
potential energy value stored by the :doc:`compute pe <compute_pe>`
|
|
compute, thinking it is current. There are many other commands which
|
|
could alter the state of the system between runs, causing a variable
|
|
to evaluate incorrectly.
|
|
|
|
The solution to this issue is the same as for case (2) above, namely
|
|
perform a 0-timestep run before the variable is evaluated to insure
|
|
the system is up-to-date. For example, this sequence of commands
|
|
would print a potential energy that reflected the changed pairwise
|
|
coefficient:
|
|
|
|
.. parsed-literal::
|
|
|
|
pair_coeff 1 1 1.0 1.0
|
|
run 1000
|
|
pair_coeff 1 1 1.5 1.0
|
|
run 0
|
|
variable e equal pe
|
|
print "Final potential energy = $e"
|
|
|
|
|
|
----------
|
|
|
|
|
|
Restrictions
|
|
""""""""""""
|
|
|
|
|
|
Indexing any formula element by global atom ID, such as an atom value,
|
|
requires the atom style to use a global mapping in order to look up
|
|
the vector indices. By default, only atom styles with molecular
|
|
information create global maps. The :doc:`atom_modify map <atom_modify>` command can override the default.
|
|
|
|
All *universe*- and *uloop*-style variables defined in an input script
|
|
must have the same number of values.
|
|
|
|
Related commands
|
|
""""""""""""""""
|
|
|
|
:doc:`next <next>`, :doc:`jump <jump>`, :doc:`include <include>`,
|
|
:doc:`temper <temper>`, :doc:`fix print <fix_print>`, :doc:`print <print>`
|
|
|
|
**Default:** none
|
|
|
|
|
|
.. _lws: http://lammps.sandia.gov
|
|
.. _ld: Manual.html
|
|
.. _lc: Section_commands.html#comm
|