"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c

:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)

:line

compute sum command :h3

[Syntax:]

compute ID group-ID sum value1 value2 ... :pre

ID, group-ID are documented in "compute"_compute.html command :ulb,l
sum = style name of this compute command :l
one or more values can be listed :l
value = x, y, z, vx, vy, vz, fx, fy, fz, c_ID, c_ID\[N\], f_ID, f_ID\[N\], v_name :l
  x,y,z,vx,vy,vz,fx,fy,fz = atom attribute (position, velocity, force component)
  c_ID = per-atom scalar value calculated by a compute with ID
  c_ID\[N\] = Nth component of per-atom vector calculated by a compute with ID
  f_ID = per-atom scalar value calculated by a fix with ID
  f_ID\[N\] = Nth component of per-atom vector calculated by a fix with ID
  v_name = per-atom value calculated by an atom-style variable with name :pre
:ule

[Examples:]

compute 1 all sum c_force
compute 2 all sum c_press[2] f_ave v_myKE :pre

[Description:]

Define a calculation that sums the results of one or more per-atom
quantities across all atoms in the group to yield a global scalar or
vector quantity.  The resulting value(s) can be accessed by any
command that uses global computes, e.g. the "thermo
custom"_therml_style.html command or "fix ave/time"_fix_ave_time.html
command or by a "variable"_variable.html command.  See "this
section"_Section_howto.html#4_15 of the documentation for an overview
of output options.

Each listed value is summed independently.  The group specified with
the command means only atoms within the group contribute to the sum.
Note that the value that produces the per-atom quantities may define
its own group which affects the values it returns.  For example, if a
per-atom compute is used as a value, it will generate values of 0.0
for atoms that are not in the group specified for that compute.

Each listed value can be an atom attribute (position, velocity, force
component) or can be the result of a "compute"_compute.html or
"fix"_fix.html or the evaluation of an atom-style
"variable"_variable.html.  In the latter cases, the compute, fix, or
variable must produce a per-atom quantity, not a global quantity.

"Computes"_compute.html that produce per-atom quantities are those
which have the word {atom} in their style name.  See the doc pages for
individual "fixes"_fix.html to determine which ones produce per-atom
quantities.  "Variables"_variable.html of style {atom} are the only
ones that can be used with this compute since all other variable
styles produce global quantities.

If a single value is specified this compute produces a global scalar
value.  If multiple values are specified, this compute produces a
vector of global values, the length of which is equal to the
number of values specified.

The value(s) produced by this compute are all "extensive", meaning
their value scales linearly with the number of atoms involved.  If
normalized values are desired, this compute can be accessed by the
"thermo_style custom"_thermo_style.html command with "thermo_modify
norm yes"_thermo_modify.html set as an option.  Or it can be accessed
by a "variable"_variable.html that divides by the appropriate atom
count.

[Output info:]

The scalar and vector values calculated by this compute are
"extensive", meaning they scale with the number of atoms in the
simulation.

[Restrictions:] none

[Related commands:]

"compute"_compute.html, "fix"_fix.html, "variable"_variable.html

[Default:] none