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