forked from lijiext/lammps
118 lines
4.5 KiB
Plaintext
118 lines
4.5 KiB
Plaintext
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"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/piston command :h3
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[Syntax:]
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fix ID group-ID wall/piston face arg ... keyword value ... :pre
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ID, group-ID are documented in "fix"_fix.html command :ulb,l
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wall/piston = style name of this fix command :l
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one face/arg pair must be appended :l
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face = {zlo} :l
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zero or more keyword/value pairs may be appended :l
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keyword = {pos} or {vel} or {ramp} or {units} :l
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{pos} args = z
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z = z coordinate at which the piston begins (distance units)
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{vel} args = vz
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vz = final velocity of the piston (velocity units)
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{ramp} = use a linear velocity ramp from 0 to vz
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{temp} args = target damp seed extent
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target = target velocity for region immediately ahead of the piston
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damp = damping paramter (time units)
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seed = random number seed for langevin kicks
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extent = extent of thermostated region (distance units)
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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 :pre
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:ule
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[Examples:]
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fix xwalls all wall/piston zlo
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fix walls all wall/piston zlo pos 1.0 0.0 0.0 vel 0.0 0.0 10.0 units box
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fix top all wall/piston zlo vel 0.0 0.0 10.0 ramp :pre
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[Description:]
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Bound the simulation with a moving wall which reflect particles
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in the specified group and drive the system with an effective infinite-mass
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piston capable of driving shock waves.
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A momentum mirror technique is used, which means that if an atom (or the wall)
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moves such that an atom is outside the wall on a timestep
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by a distance delta (e.g. due to "fix nve"_fix_nve.html), then it is
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put back inside the face by the same delta, and the velocity relative to
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the moving wall is flipped in z. For instance, a stationary particle hit with a
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piston wall with velocity vz, will end the timestep with a velocity of 2*vz.
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Currently only face {zlo} may be specified.
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This creates a piston moving in the positive z direction.
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Particles with z coordinate less than the wall position
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are reflected to a z coordinate
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greater than the wall position.
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If the piston velocity is vpz and the particle velocity
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before reflection is vzi, the particle velocity after
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reflection is -vzi + 2*vpz.
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The initial position of the wall can be specified by the {pos} keyword.
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The final velocity of the wall can be specified by the {vel} keyword
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The {ramp} keyword will cause the wall/piston to adjust the velocity linearly
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from zero velocity to {vel} over the course of the run. If the {ramp} keyword is omitted
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then the wall/piston moves at a constant velocity defined by {vel}.
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The {temp} keyword will cause the region immediately in front of the wall/piston
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to be thermostated with a Langevin thermostat. This region moves with the piston.
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The damping and kicking are measured in the reference frame of the piston.
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So, a temperature of zero would mean all particles were moving at exactly the speed
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of the wall/piston.
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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 is used.
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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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: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. None of the "fix_modify"_fix_modify.html options
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are relevant to this fix. No global or per-atom quantities are stored
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by this fix for access by various "output
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commands"_Section_howto.html#4_15. No parameter of this fix can be
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used with the {start/stop} keywords of the "run"_run.html command.
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This fix is not invoked during "energy minimization"_minimize.html.
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[Restrictions:]
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The face that has the wall/piston must be boundary type 'm' (shrink-wrapped
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with minimum a minimum value). The opposing face can be
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any boundary type other than periodic.
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A wall/piston 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/piston
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displaces atoms directly rather than exerting a force on them.
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[Related commands:]
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"fix wall/reflect"_fix_wall.html command
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"fix append/atoms"_fix_append_atoms.html command
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[Default:] {pos} = 0, {vel} = 0, {units} = lattice.
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:line
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