<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
a distance delta (e.g. due to <aclass="reference internal"href="fix_nve.html"><spanclass="doc">fix nve</span></a>), then it is put
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
<aclass="reference internal"href="units.html"><spanclass="doc">units</span></a> command, e.g. Angstroms for units = real or metal.
A <em>lattice</em> value means the distance units are in lattice spacings.
The <aclass="reference internal"href="lattice.html"><spanclass="doc">lattice</span></a> command must have been previously used to
define the lattice spacings.</p>
<hrclass="docutils"/>
<p><strong>Restart, fix_modify, output, run start/stop, minimize info:</strong></p>
<p>No information about this fix is written to <aclass="reference internal"href="restart.html"><spanclass="doc">binary restart files</span></a>. None of the <aclass="reference internal"href="fix_modify.html"><spanclass="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 <aclass="reference internal"href="Section_howto.html#howto-15"><spanclass="std std-ref">output commands</span></a>. No parameter of this fix can
be used with the <em>start/stop</em> keywords of the <aclass="reference internal"href="run.html"><spanclass="doc">run</span></a> command.
This fix is not invoked during <aclass="reference internal"href="minimize.html"><spanclass="doc">energy minimization</span></a>.</p>
</div>
<divclass="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 <aclass="reference internal"href="Section_start.html#start-3"><spanclass="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>
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