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<div class="section" id="fix-wall-piston-command">
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<span id="index-0"></span><h1>fix wall/piston command</h1>
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<div class="section" id="syntax">
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<h2>Syntax</h2>
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<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>
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</pre></div>
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</div>
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<ul class="simple">
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<li>ID, group-ID are documented in <a class="reference internal" href="fix.html"><span class="doc">fix</span></a> command</li>
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<li>wall/piston = style name of this fix command</li>
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<li>face = <em>zlo</em></li>
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<li>zero or more keyword/value pairs may be appended</li>
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<li>keyword = <em>pos</em> or <em>vel</em> or <em>ramp</em> or <em>units</em></li>
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</ul>
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<pre class="literal-block">
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<em>pos</em> args = z
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z = z coordinate at which the piston begins (distance units)
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<em>vel</em> args = vz
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vz = final velocity of the piston (velocity units)
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<em>ramp</em> = use a linear velocity ramp from 0 to vz
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<em>temp</em> 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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<em>units</em> value = <em>lattice</em> or <em>box</em>
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<em>lattice</em> = the wall position is defined in lattice units
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<em>box</em> = the wall position is defined in simulation box units
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</pre>
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</div>
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<div class="section" id="examples">
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<h2>Examples</h2>
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<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>
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<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>
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<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>
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</pre></div>
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</div>
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</div>
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<div class="section" id="description">
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<h2>Description</h2>
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<p>Bound the simulation with a moving wall which reflect particles in the
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specified group and drive the system with an effective infinite-mass
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piston capable of driving shock waves.</p>
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<p>A momentum mirror technique is used, which means that if an atom (or
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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
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the moving wall is flipped in z. For instance, a stationary particle
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hit with a piston wall with velocity vz, will end the timestep with a
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velocity of 2*vz.</p>
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<p>Currently the <em>face</em> keyword can only be <em>zlo</em>. This creates a piston
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moving in the positive z direction. Particles with z coordinate less
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than the wall position are reflected to a z coordinate greater than
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the wall position. If the piston velocity is vpz and the particle
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velocity before reflection is vzi, the particle velocity after
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reflection is -vzi + 2*vpz.</p>
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<p>The initial position of the wall can be specified by the <em>pos</em> keyword.</p>
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<p>The final velocity of the wall can be specified by the <em>vel</em> keyword</p>
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<p>The <em>ramp</em> keyword will cause the wall/piston to adjust the velocity
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linearly from zero velocity to <em>vel</em> over the course of the run. If
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the <em>ramp</em> keyword is omitted then the wall/piston moves at a constant
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velocity defined by <em>vel</em>.</p>
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<p>The <em>temp</em> keyword will cause the region immediately in front of the
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wall/piston to be thermostated with a Langevin thermostat. This
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region moves with the piston. The damping and kicking are measured in
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the reference frame of the piston. So, a temperature of zero would
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mean all particles were moving at exactly the speed of the
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wall/piston.</p>
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<p>The <em>units</em> 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.</p>
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<p>A <em>box</em> value selects standard distance units as defined by the
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<a class="reference internal" href="units.html"><span class="doc">units</span></a> 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.
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The <a class="reference internal" href="lattice.html"><span class="doc">lattice</span></a> command must have been previously used to
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define the lattice spacings.</p>
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</div>
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<hr class="docutils" />
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<div class="section" id="restart-fix-modify-output-run-start-stop-minimize-info">
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<h2>Restart, fix_modify, output, run start/stop, minimize info</h2>
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<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
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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 <span class="xref std std-ref">output commands</span>. No parameter of this fix can
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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.
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This fix is not invoked during <a class="reference internal" href="minimize.html"><span class="doc">energy minimization</span></a>.</p>
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</div>
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<div class="section" id="restrictions">
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<h2>Restrictions</h2>
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<p>This fix style is part of the SHOCK package. It is only enabled if
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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>
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<p>The face that has the wall/piston must be boundary type ‘s’
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(shrink-wrapped). The opposing face can be
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any boundary type other than periodic.</p>
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<p>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.</p>
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</div>
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<div class="section" id="related-commands">
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<h2>Related commands</h2>
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<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>
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</div>
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<div class="section" id="default">
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<h2>Default</h2>
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<p>The keyword defaults are pos = 0, vel = 0, units = lattice.</p>
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