forked from lijiext/lammps
293 lines
13 KiB
Plaintext
293 lines
13 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Section_commands.html#comm)
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:line
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fix wall/lj93 command :h3
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fix wall/lj126 command :h3
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fix wall/lj1043 command :h3
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fix wall/colloid command :h3
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fix wall/harmonic command :h3
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[Syntax:]
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fix ID group-ID style face args ... keyword value ... :pre
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ID, group-ID are documented in "fix"_fix.html command :ulb,l
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style = {wall/lj93} or {wall/lj126} or {wall/lj1043} or {wall/colloid} or {wall/harmonic} :l
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one or more face/arg pairs may be appended :l
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face = {xlo} or {xhi} or {ylo} or {yhi} or {zlo} or {zhi} :l
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args = coord epsilon sigma cutoff
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coord = position of wall = EDGE or constant or variable
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EDGE = current lo or hi edge of simulation box
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constant = number like 0.0 or -30.0 (distance units)
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variable = "equal-style variable"_variable.html like v_x or v_wiggle
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epsilon = strength factor for wall-particle interaction (energy or energy/distance^2 units)
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epsilon can be a variable (see below)
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sigma = size factor for wall-particle interaction (distance units)
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sigma can be a variable (see below)
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cutoff = distance from wall at which wall-particle interaction is cut off (distance units) :pre
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zero or more keyword/value pairs may be appended :l
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keyword = {units} or {fld} :l
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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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{fld} value = {yes} or {no}
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{yes} = invoke the wall constraint to be compatible with implicit FLD
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{no} = invoke the wall constraint in the normal way
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{pbc} value = {yes} or {no}
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{yes} = allow periodic boundary in a wall dimension
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{no} = require non-perioidic boundaries in any wall dimension :pre
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:ule
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[Examples:]
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fix wallhi all wall/lj93 xlo -1.0 1.0 1.0 2.5 units box
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fix wallhi all wall/lj93 xhi EDGE 1.0 1.0 2.5
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fix wallhi all wall/lj126 v_wiggle 23.2 1.0 1.0 2.5
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fix zwalls all wall/colloid zlo 0.0 1.0 1.0 0.858 zhi 40.0 1.0 1.0 0.858 :pre
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[Description:]
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Bound the simulation domain on one or more of its faces with a flat
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wall that interacts with the atoms in the group by generating a force
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on the atom in a direction perpendicular to the wall. The energy of
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wall-particle interactions depends on the style.
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For style {wall/lj93}, the energy E is given by the 9/3 potential:
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:c,image(Eqs/fix_wall_lj93.jpg)
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For style {wall/lj126}, the energy E is given by the 12/6 potential:
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:c,image(Eqs/pair_lj.jpg)
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For style {wall/lj1043}, the energy E is given by the 10/4/3 potential:
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:c,image(Eqs/fix_wall_lj1043.jpg)
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For style {wall/colloid}, the energy E is given by an integrated form
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of the "pair_style colloid"_pair_colloid.html potential:
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:c,image(Eqs/fix_wall_colloid.jpg)
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For style {wall/harmonic}, the energy E is given by a harmonic spring
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potential:
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:c,image(Eqs/fix_wall_harmonic.jpg)
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In all cases, {r} is the distance from the particle to the wall at
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position {coord}, and Rc is the {cutoff} distance at which the
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particle and wall no longer interact. The energy of the wall
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potential is shifted so that the wall-particle interaction energy is
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0.0 at the cutoff distance.
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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 interacts with
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particles near the lower side of the simulation box in that dimension.
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A {hi} face interacts with particles near the upper side of the
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simulation box in that dimension.
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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 "equal-style variable"_variable.html 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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"thermo_style"_thermo_style.html 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. See examples below.
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For the {wall/lj93} and {wall/lj126} and {wall/lj1043} styles,
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{epsilon} and {sigma} are the usual Lennard-Jones parameters, which
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determine the strength and size of the particle as it interacts with
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the wall. Epsilon has energy units. Note that this {epsilon} and
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{sigma} may be different than any {epsilon} or {sigma} values defined
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for a pair style that computes particle-particle interactions.
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The {wall/lj93} interaction is derived by integrating over a 3d
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half-lattice of Lennard-Jones 12/6 particles. The {wall/lj126}
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interaction is effectively a harder, more repulsive wall interaction.
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The {wall/lj1043} interaction is yet a different form of wall
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interaction, described in Magda et al in "(Magda)"_#Magda.
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For the {wall/colloid} style, {R} is the radius of the colloid
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particle, {D} is the distance from the surface of the colloid particle
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to the wall (r-R), and {sigma} is the size of a constituent LJ
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particle inside the colloid particle and wall. Note that the cutoff
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distance Rc in this case is the distance from the colloid particle
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center to the wall. The prefactor {epsilon} can be thought of as an
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effective Hamaker constant with energy units for the strength of the
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colloid-wall interaction. More specifically, the {epsilon} pre-factor
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= 4 * pi^2 * rho_wall * rho_colloid * epsilon * sigma^6, where epsilon
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and sigma are the LJ parameters for the constituent LJ
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particles. Rho_wall and rho_colloid are the number density of the
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constituent particles, in the wall and colloid respectively, in units
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of 1/volume.
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The {wall/colloid} interaction is derived by integrating over
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constituent LJ particles of size {sigma} within the colloid particle
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and a 3d half-lattice of Lennard-Jones 12/6 particles of size {sigma}
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in the wall. As mentioned in the preceeding paragraph, the density of
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particles in the wall and colloid can be different, as specified by
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the {epsilon} pre-factor.
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For the {wall/harmonic} style, {epsilon} is effectively the spring
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constant K, and has units (energy/distance^2). The input parameter
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{sigma} is ignored. The minimum energy position of the harmonic
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spring is at the {cutoff}. This is a repulsive-only spring since the
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interaction is truncated at the {cutoff}
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For any wall, the {epsilon} and/or {sigma} parameter can be specified
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as an "equal-style variable"_variable.html, in which case it should be
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specified as v_name, where name is the variable name. As with a
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variable wall position, the variable is evaluated each timestep and
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the result becomes the current epsilon or sigma of the wall.
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Equal-style variables can specify formulas with various mathematical
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functions, and include "thermo_style"_thermo_style.html command
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keywords for the simulation box parameters and timestep and elapsed
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time. Thus it is easy to specify a time-dependent wall interaction.
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IMPORTANT NOTE: For all of the styles, you must insure that r is
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always > 0 for all particles in the group, or LAMMPS will generate an
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error. This means you cannot start your simulation with particles at
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the wall position {coord} (r = 0) or with particles on the wrong side
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of the wall (r < 0). For the {wall/lj93} and {wall/lj126} styles, the
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energy of the wall/particle interaction (and hence the force on the
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particle) blows up as r -> 0. The {wall/colloid} style is even more
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restrictive, since the energy blows up as D = r-R -> 0. This means
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the finite-size particles of radius R must be a distance larger than R
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from the wall position {coord}. The {harmonic} style is a softer
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potential and does not blow up as r -> 0, but you must use a large
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enough {epsilon} that particles always reamin on the correct side of
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the wall (r > 0).
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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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"units"_units.html 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 "lattice"_lattice.html command must have been previously used to
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define the lattice spacings.
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The {fld} keyword can be used with a {yes} setting to invoke the wall
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constraint before pairwise interactions are computed. This allows an
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implicit FLD model using "pair_style lubricateU"_pair_lubricateU.html
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to include the wall force in its calculations. If the setting is
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{no}, wall forces are imposed after pairwise interactions, in the
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usual manner.
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The {pbc} keyword can be used with a {yes} setting to allow walls to
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be specified in a periodic dimension. See the
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"boundary"_boundary.html command for options on simulation box
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boundaries. The default for {pbc} is {no}, which means the system
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must be non-periodic when using a wall. But you may wish to use a
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periodic box. E.g. to allow some particles to interact with the wall
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via the fix group-ID, and others to pass through it and wrap around a
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periodic box. In this case you should insure that the wall if
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sufficiently far enough away from the box boundary. If you do not,
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then particles may interact with both the wall and with periodic
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images on the other side of the box, which is probably not what you
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want.
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:line
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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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"variables"_variable.html. The wall interaction parameters (epsilon,
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sigma) could be varied with additional variable definitions.
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variable ramp equal ramp(0,10)
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fix 1 all wall xlo v_ramp 1.0 1.0 2.5 :pre
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variable linear equal vdisplace(0,20)
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fix 1 all wall xlo v_linear 1.0 1.0 2.5 :pre
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variable wiggle equal swiggle(0.0,5.0,3.0)
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fix 1 all wall xlo v_wiggle 1.0 1.0 2.5 :pre
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variable wiggle equal cwiggle(0.0,5.0,3.0)
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fix 1 all wall xlo v_wiggle 1.0 1.0 2.5 :pre
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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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position = c0 + A sin(omega*delta) :pre
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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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position = c0 + A (1 - cos(omega*delta)) :pre
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:line
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[Restart, fix_modify, output, run start/stop, minimize info:]
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No information about this fix is written to "binary restart
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files"_restart.html.
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The "fix_modify"_fix_modify.html {energy} option is supported by this
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fix to add the energy of interaction between atoms and each wall to
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the system's potential energy as part of "thermodynamic
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output"_thermo_style.html.
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This fix computes a global scalar energy and a global vector of
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forces, which can be accessed by various "output
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commands"_Section_howto.html#howto_15. Note that the scalar energy is
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the sum of interactions with all defined walls. If you want the
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energy on a per-wall basis, you need to use multiple fix wall
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commands. The length of the vector is equal to the number of walls
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defined by the fix. Each vector value is the normal force on a
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specific wall. Note that an outward force on a wall will be a
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negative value for {lo} walls and a positive value for {hi} walls.
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The scalar and vector values calculated by this fix are "extensive".
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No parameter of this fix can be used with the {start/stop} keywords of
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the "run"_run.html command.
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The forces due to this fix are imposed during an energy minimization,
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invoked by the "minimize"_minimize.html command.
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IMPORTANT NOTE: If you want the atom/wall interaction energy to be
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included in the total potential energy of the system (the quantity
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being minimized), you MUST enable the "fix_modify"_fix_modify.html
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{energy} option for this fix.
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[Restrictions:] none
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[Related commands:]
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"fix wall/reflect"_fix_wall_reflect.html,
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"fix wall/gran"_fix_wall_gran.html,
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"fix wall/region"_fix_wall_region.html
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[Default:]
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The option defaults units = lattice, fld = no, and pbc = no.
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:line
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:link(Magda)
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[(Magda)] Magda, Tirrell, Davis, J Chem Phys, 83, 1888-1901 (1985);
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erratum in JCP 84, 2901 (1986).
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