forked from lijiext/lammps
169 lines
7.0 KiB
Plaintext
169 lines
7.0 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/colloid command :h3
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[Syntax:]
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fix ID group-ID style keyword values ... :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/colloid} :l
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one or more keyword/value pairs may be appended :l
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keyword = {xlo} or {xhi} or {ylo} or {yhi} or {zlo} or {zhi} or {vel} or {wiggle} :l
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{xlo}, {xhi}, {ylo}, {yhi}, {zlo}, {zhi} values = coord epsilon sigma cutoff
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coord = position of wall (distance units)
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epsilon = strength factor for wall-particle interaction (energy units)
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sigma = size factor for wall-particle interaction (distance units)
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cutoff = distance from wall at which wall-particle interaction is cut off (distance units)
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{vel} value = v
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v = velocity of wall in perpendicular direction (velocity units)
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{wiggle} values = amplitude period
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amplitude = size of oscillation (distance units)
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period = time of oscillation (time units) :pre
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:ule
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[Examples:]
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fix wallhi all wall/lj93 xhi 10.0 1.0 1.0 2.5
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fix wallhi all wall/lj126 xhi 10.0 1.0 1.0 2.5 vel 1.0
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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/colloid}, the energy E is given by an integrated form of
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the "pair_style colloid"_pair_colloid.html potential:
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:c,image(Eqs/fix_wall_colloid.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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For the {wall/lj93} and {wall/lj126} styles, {epsilon} and {sigma} are
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the usual Lennard-Jones parameters, which determine the strength and
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size of the particle as it interacts with the wall. Note that this
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{epsilon} and {sigma} may be different than any {epsilon} or {sigma}
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values defined for a pair style that computes particle-particle
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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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For the {wall/colloid} style, {epsilon} is effectively a Hamaker
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constant for the colloid-wall interaction, {R} is the radius of the
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colloid particle, {D} is the distance from the surface of the colloid
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particle to the wall (r-R), and {sigma} is the size of the constituent
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LJ particle inside the colloid particle. 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.
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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.
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If the {vel} keyword is specified, the position of all walls will move
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during the simulation in a perpendicular direction, based on their
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initial {coord} position, the specified velocity {vel}, and the time
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elapsed since the beginning of the simulation. See the note below
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about making a wall move continuously across multiple runs. A
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positive velocity means each wall moves inward, towards the center of
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the box. I.e. an {xlo} wall will move in the +x direction and an
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{xhi} wall will move in the -x direction. A negative velocity means
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each wall moves outward, away from the center of the box. If you want
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different walls to move with different velocities, then you need to
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use multiple fix wall commands.
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If the {wiggle} keyword is specified, the position of all walls will
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oscillate sinusoidally during the simulation in the perpendicular
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direction, according to the equation:
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position = pos0 + A sin (omega * delta) :pre
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where {pos0} is the position at the beginning of the simulation, {A}
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is the {amplitude}, {omega} is 2 PI / {period}, and {delta} is the
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time elapsed since the beginning of the simulation. See the note
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below about making a wall oscillate continuously across multiple runs.
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A positive amplitude means each wall initially moves inward, towards
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the center of the box. I.e. an {xlo} wall will move initially in the
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+x direction and an {xhi} wall will move initially in the -x
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direction. A negative velocity means each wall moves initially
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outward, away from the center of the box. If you want different walls
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to oscillate with different amplitudes or periods, then you need to
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use multiple fix wall commands.
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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 scalar energy and a 6-length vector of forces (one
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force magnitude per wall), which can be accessed by various "output
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commands"_Section_howto.html#4_15. The scalar and vector values
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calculated by this fix are "extensive", meaning they scale with the
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number of atoms in the simulation. Note that the scalar energy is the
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sum of interactions with all defined walls. If you want the energy on
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a per-wall basis, you need to use multiple fix wall commands. The 6
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vector quantities are the force on the {xlo} wall, the {xhi} wall,
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{ylo}, {yhi}, {zlo}, {zhi}. These values will only be non-zero if the
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corresponding wall is defined. Note that an outward force on a wall
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will be a negative value for {lo} walls and a positive value for {hi}
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walls.
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This fix can change the position of each wall, due to the {vel} or
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{wiggle} keywords, continuously over multiple runs, using the {start}
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and {stop} keywords of the "run"_run.html command. If you do not do
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this, the wall position will be reset to {coord} at the beginning of
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each run.
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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:]
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Any dimension (xyz) that has a wall must be non-periodic.
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You cannot use both the {vel} and {wiggle} keywords together.
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[Related commands:]
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"fix wall/reflect"_fix_wall_reflect.html wall/gran"_fix_wall_gran.html
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[Default:]
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The option defaults are no velocity and no wiggle.
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