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< li > fix wall/piston command< / li >
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< div class = "section" id = "fix-wall-piston-command" >
< span id = "index-0" > < / span > < h1 > fix wall/piston command< / h1 >
< div class = "section" id = "syntax" >
< h2 > Syntax< / h2 >
< div class = "highlight-default" > < div class = "highlight" > < pre > < span > < / span > < span class = "n" > fix< / span > < span class = "n" > ID< / span > < span class = "n" > group< / span > < span class = "o" > -< / span > < span class = "n" > ID< / span > < span class = "n" > wall< / span > < span class = "o" > /< / span > < span class = "n" > piston< / span > < span class = "n" > face< / span > < span class = "o" > ...< / span > < span class = "n" > keyword< / span > < span class = "n" > value< / span > < span class = "o" > ...< / span >
< / pre > < / div >
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< ul class = "simple" >
< li > ID, group-ID are documented in < a class = "reference internal" href = "fix.html" > < span class = "doc" > fix< / span > < / a > command< / li >
< li > wall/piston = style name of this fix command< / li >
< li > face = < em > zlo< / em > < / li >
< li > zero or more keyword/value pairs may be appended< / li >
< li > keyword = < em > pos< / em > or < em > vel< / em > or < em > ramp< / em > or < em > units< / em > < / li >
< / ul >
< pre class = "literal-block" >
< em > pos< / em > args = z
z = z coordinate at which the piston begins (distance units)
< em > vel< / em > args = vz
vz = final velocity of the piston (velocity units)
< em > ramp< / em > = use a linear velocity ramp from 0 to vz
< em > temp< / em > args = target damp seed extent
target = target velocity for region immediately ahead of the piston
damp = damping paramter (time units)
seed = random number seed for langevin kicks
extent = extent of thermostated region (distance units)
< em > units< / em > value = < em > lattice< / em > or < em > box< / em >
< em > lattice< / em > = the wall position is defined in lattice units
< em > box< / em > = the wall position is defined in simulation box units
< / pre >
< / div >
< div class = "section" id = "examples" >
< h2 > Examples< / h2 >
< div class = "highlight-default" > < div class = "highlight" > < pre > < span > < / span > < span class = "n" > fix< / span > < span class = "n" > xwalls< / span > < span class = "nb" > all< / span > < span class = "n" > wall< / span > < span class = "o" > /< / span > < span class = "n" > piston< / span > < span class = "n" > zlo< / span >
< span class = "n" > fix< / span > < span class = "n" > walls< / span > < span class = "nb" > all< / span > < span class = "n" > wall< / span > < span class = "o" > /< / span > < span class = "n" > piston< / span > < span class = "n" > zlo< / span > < span class = "n" > pos< / span > < span class = "mf" > 1.0< / span > < span class = "n" > vel< / span > < span class = "mf" > 10.0< / span > < span class = "n" > units< / span > < span class = "n" > box< / span >
< span class = "n" > fix< / span > < span class = "n" > top< / span > < span class = "nb" > all< / span > < span class = "n" > wall< / span > < span class = "o" > /< / span > < span class = "n" > piston< / span > < span class = "n" > zlo< / span > < span class = "n" > vel< / span > < span class = "mf" > 10.0< / span > < span class = "n" > ramp< / span >
< / pre > < / div >
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< div class = "section" id = "description" >
< h2 > Description< / h2 >
< p > Bound the simulation with a moving wall which reflect particles in the
specified group and drive the system with an effective infinite-mass
piston capable of driving shock waves.< / p >
< p > A momentum mirror technique is used, which means that if an atom (or
the wall) moves such that an atom is outside the wall on a timestep by
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a distance delta (e.g. due to < a class = "reference internal" href = "fix_nve.html" > < span class = "doc" > fix nve< / span > < / a > ), then it is put
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back inside the face by the same delta, and the velocity relative to
the moving wall is flipped in z. For instance, a stationary particle
hit with a piston wall with velocity vz, will end the timestep with a
velocity of 2*vz.< / p >
< p > Currently the < em > face< / em > keyword can only be < em > zlo< / em > . This creates a piston
moving in the positive z direction. Particles with z coordinate less
than the wall position are reflected to a z coordinate greater than
the wall position. If the piston velocity is vpz and the particle
velocity before reflection is vzi, the particle velocity after
reflection is -vzi + 2*vpz.< / p >
< p > The initial position of the wall can be specified by the < em > pos< / em > keyword.< / p >
< p > The final velocity of the wall can be specified by the < em > vel< / em > keyword< / p >
< p > The < em > ramp< / em > keyword will cause the wall/piston to adjust the velocity
linearly from zero velocity to < em > vel< / em > over the course of the run. If
the < em > ramp< / em > keyword is omitted then the wall/piston moves at a constant
velocity defined by < em > vel< / em > .< / p >
< p > The < em > temp< / em > keyword will cause the region immediately in front of the
wall/piston to be thermostated with a Langevin thermostat. This
region moves with the piston. The damping and kicking are measured in
the reference frame of the piston. So, a temperature of zero would
mean all particles were moving at exactly the speed of the
wall/piston.< / p >
< p > The < em > units< / em > keyword determines the meaning of the distance units used
to define a wall position, but only when a numeric constant is used.< / p >
< p > A < em > box< / em > value selects standard distance units as defined by the
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< span class = "xref doc" > units< / span > command, e.g. Angstroms for units = real or metal.
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A < em > lattice< / em > value means the distance units are in lattice spacings.
The < a class = "reference internal" href = "lattice.html" > < span class = "doc" > lattice< / span > < / a > command must have been previously used to
define the lattice spacings.< / p >
< / div >
< hr class = "docutils" / >
< div class = "section" id = "restart-fix-modify-output-run-start-stop-minimize-info" >
< h2 > Restart, fix_modify, output, run start/stop, minimize info< / h2 >
< p > No information about this fix is written to < a class = "reference internal" href = "restart.html" > < span class = "doc" > binary restart files< / span > < / a > . None of the < a class = "reference internal" href = "fix_modify.html" > < span class = "doc" > fix_modify< / span > < / a > options
are relevant to this fix. No global or per-atom quantities are stored
by this fix for access by various < span class = "xref std std-ref" > output commands< / span > . No parameter of this fix can
be used with the < em > start/stop< / em > keywords of the < a class = "reference internal" href = "run.html" > < span class = "doc" > run< / span > < / a > command.
This fix is not invoked during < a class = "reference internal" href = "minimize.html" > < span class = "doc" > energy minimization< / span > < / a > .< / p >
< / div >
< div class = "section" id = "restrictions" >
< h2 > Restrictions< / h2 >
< p > This fix style is part of the SHOCK package. It is only enabled if
LAMMPS was built with that package. See the < a class = "reference internal" href = "Section_start.html#start-3" > < span class = "std std-ref" > Making LAMMPS< / span > < / a > section for more info.< / p >
< p > The face that has the wall/piston must be boundary type ‘ s’
(shrink-wrapped). The opposing face can be
any boundary type other than periodic.< / p >
< p > A wall/piston should not be used with rigid bodies such as those
defined by a “ fix rigid” command. This is because the wall/piston
displaces atoms directly rather than exerting a force on them.< / p >
< / div >
< div class = "section" id = "related-commands" >
< h2 > Related commands< / h2 >
< p > < a class = "reference internal" href = "fix_wall.html" > < span class = "doc" > fix wall/reflect< / span > < / a > command, < a class = "reference internal" href = "fix_append_atoms.html" > < span class = "doc" > fix append/atoms< / span > < / a > command< / p >
< / div >
< div class = "section" id = "default" >
< h2 > Default< / h2 >
< p > The keyword defaults are pos = 0, vel = 0, units = lattice.< / p >
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