185 lines
6.5 KiB
ReStructuredText
185 lines
6.5 KiB
ReStructuredText
.. index:: fix wall/reflect
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.. index:: fix wall/reflect/kk
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fix wall/reflect command
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========================
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Accelerator Variants: *wall/reflect/kk*
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Syntax
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""""""
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.. parsed-literal::
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fix ID group-ID wall/reflect face arg ... keyword value ...
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* wall/reflect = style name of this fix command
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* one or more face/arg pairs may be appended
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* face = *xlo* or *xhi* or *ylo* or *yhi* or *zlo* or *zhi*
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.. parsed-literal::
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arg = EDGE or constant or variable
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EDGE = current lo edge of simulation box
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constant = number like 0.0 or 30.0 (distance units)
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variable = :doc:`equal-style variable <variable>` like v_x or v_wiggle
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* zero or more keyword/value pairs may be appended
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* keyword = *units*
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.. parsed-literal::
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*units* value = *lattice* or *box*
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*lattice* = the wall position is defined in lattice units
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*box* = the wall position is defined in simulation box units
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Examples
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""""""""
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.. code-block:: LAMMPS
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fix xwalls all wall/reflect xlo EDGE xhi EDGE
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fix walls all wall/reflect xlo 0.0 ylo 10.0 units box
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fix top all wall/reflect zhi v_pressdown
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Description
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"""""""""""
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Bound the simulation with one or more walls which reflect particles
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in the specified group when they attempt to move through them.
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Reflection means that if an atom moves outside the wall on a timestep
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by a distance delta (e.g. due to :doc:`fix nve <fix_nve>`), then it is
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put back inside the face by the same delta, and the sign of the
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corresponding component of its velocity is flipped.
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When used in conjunction with :doc:`fix nve <fix_nve>` and
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:doc:`run_style verlet <run_style>`, the resultant time-integration
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algorithm is equivalent to the primitive splitting algorithm (PSA)
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described by :ref:`Bond <Bond1>`. Because each reflection event
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divides the corresponding timestep asymmetrically, energy conservation
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is only satisfied to O(dt), rather than to O(dt\^2) as it would be for
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velocity-Verlet integration without reflective walls.
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Up to 6 walls or faces can be specified in a single command: *xlo*\ ,
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*xhi*\ , *ylo*\ , *yhi*\ , *zlo*\ , *zhi*\ . A *lo* face reflects particles
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that move to a coordinate less than the wall position, back in the
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*hi* direction. A *hi* face reflects particles that move to a
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coordinate higher than the wall position, back in the *lo* direction.
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The position of each wall can be specified in one of 3 ways: as the
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EDGE of the simulation box, as a constant value, or as a variable. If
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EDGE is used, then the corresponding boundary of the current
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simulation box is used. If a numeric constant is specified then the
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wall is placed at that position in the appropriate dimension (x, y, or
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z). In both the EDGE and constant cases, the wall will never move.
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If the wall position is a variable, it should be specified as v_name,
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where name is an :doc:`equal-style variable <variable>` name. In this
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case the variable is evaluated each timestep and the result becomes
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the current position of the reflecting wall. Equal-style variables
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can specify formulas with various mathematical functions, and include
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:doc:`thermo_style <thermo_style>` command keywords for the simulation
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box parameters and timestep and elapsed time. Thus it is easy to
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specify a time-dependent wall position.
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The *units* keyword determines the meaning of the distance units used
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to define a wall position, but only when a numeric constant or
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variable is used. It is not relevant when EDGE is used to specify a
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face position. In the variable case, the variable is assumed to
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produce a value compatible with the *units* setting you specify.
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A *box* value selects standard distance units as defined by the
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:doc:`units <units>` command, e.g. Angstroms for units = real or metal.
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A *lattice* value means the distance units are in lattice spacings.
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The :doc:`lattice <lattice>` command must have been previously used to
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define the lattice spacings.
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----------
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Here are examples of variable definitions that move the wall position
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in a time-dependent fashion using equal-style
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:doc:`variables <variable>`.
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.. code-block:: LAMMPS
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variable ramp equal ramp(0,10)
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fix 1 all wall/reflect xlo v_ramp
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variable linear equal vdisplace(0,20)
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fix 1 all wall/reflect xlo v_linear
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variable wiggle equal swiggle(0.0,5.0,3.0)
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fix 1 all wall/reflect xlo v_wiggle
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variable wiggle equal cwiggle(0.0,5.0,3.0)
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fix 1 all wall/reflect xlo v_wiggle
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The *ramp(lo,hi)* function adjusts the wall position linearly from *lo* to
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*hi* over the course of a run. The *vdisplace(c0,velocity)* function does
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something similar using the equation *position = c0 + velocity\*delta*\ ,
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where *delta* is the elapsed time.
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The *swiggle(c0,A,period)* function causes the wall position to
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oscillate sinusoidally according to this equation, where *omega = 2 PI
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/ period*\ :
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.. parsed-literal::
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position = c0 + A sin(omega\*delta)
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The *cwiggle(c0,A,period)* function causes the wall position to
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oscillate sinusoidally according to this equation, which will have an
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initial wall velocity of 0.0, and thus may impose a gentler
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perturbation on the particles:
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.. parsed-literal::
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position = c0 + A (1 - cos(omega\*delta))
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----------
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.. include:: accel_styles.rst
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----------
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Restart, fix_modify, output, run start/stop, minimize info
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"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
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No information about this fix is written to :doc:`binary restart files
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<restart>`. None of the :doc:`fix_modify <fix_modify>` options are
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relevant to this fix. No global or per-atom quantities are stored by
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this fix for access by various :doc:`output commands <Howto_output>`.
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No parameter of this fix can be used with the *start/stop* keywords of
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the :doc:`run <run>` command. This fix is not invoked during
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:doc:`energy minimization <minimize>`.
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Restrictions
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""""""""""""
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Any dimension (xyz) that has a reflecting wall must be non-periodic.
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A reflecting wall should not be used with rigid bodies such as those
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defined by a "fix rigid" command. This is because the wall/reflect
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displaces atoms directly rather than exerts a force on them. For
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rigid bodies, use a soft wall instead, such as :doc:`fix wall/lj93
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<fix_wall>`. LAMMPS will flag the use of a rigid fix with fix
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wall/reflect with a warning, but will not generate an error.
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Related commands
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""""""""""""""""
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:doc:`fix wall/lj93 <fix_wall>`, :doc:`fix oneway <fix_oneway>`
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Default
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"""""""
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The default for the units keyword is lattice.
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----------
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.. _Bond1:
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**(Bond)** Bond and Leimkuhler, SIAM J Sci Comput, 30, p 134 (2007).
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