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<div class="section" id="fix-append-atoms-command">
<span id="index-0"></span><h1>fix append/atoms 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">append</span><span class="o">/</span><span class="n">atoms</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>
</div>
<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>append/atoms = style name of this fix command</li>
<li>face = <em>zhi</em></li>
<li>zero or more keyword/value pairs may be appended</li>
<li>keyword = <em>basis</em> or <em>size</em> or <em>freq</em> or <em>temp</em> or <em>random</em> or <em>units</em></li>
</ul>
<pre class="literal-block">
<em>basis</em> values = M itype
M = which basis atom
itype = atom type (1-N) to assign to this basis atom
<em>size</em> args = Lz
Lz = z size of lattice region appended in a single event(distance units)
<em>freq</em> args = freq
freq = the number of timesteps between append events
<em>temp</em> args = target damp seed extent
target = target temperature for the region between zhi-extent and zhi (temperature units)
damp = damping parameter (time units)
seed = random number seed for langevin kicks
extent = extent of thermostated region (distance units)
<em>random</em> args = xmax ymax zmax seed
<em>xmax</em>, <em>ymax</em>, <em>zmax</em> = maximum displacement in particular direction (distance units)
<em>seed</em> = random number seed for random displacement
<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="mi">1</span> <span class="nb">all</span> <span class="n">append</span><span class="o">/</span><span class="n">atoms</span> <span class="n">zhi</span> <span class="n">size</span> <span class="mf">5.0</span> <span class="n">freq</span> <span class="mi">295</span> <span class="n">units</span> <span class="n">lattice</span>
<span class="n">fix</span> <span class="mi">4</span> <span class="nb">all</span> <span class="n">append</span><span class="o">/</span><span class="n">atoms</span> <span class="n">zhi</span> <span class="n">size</span> <span class="mf">15.0</span> <span class="n">freq</span> <span class="mi">5</span> <span class="n">units</span> <span class="n">box</span>
<span class="n">fix</span> <span class="n">A</span> <span class="nb">all</span> <span class="n">append</span><span class="o">/</span><span class="n">atoms</span> <span class="n">zhi</span> <span class="n">size</span> <span class="mf">1.0</span> <span class="n">freq</span> <span class="mi">1000</span> <span class="n">units</span> <span class="n">lattice</span>
</pre></div>
</div>
</div>
<div class="section" id="description">
<h2>Description</h2>
<p>This fix creates atoms on a lattice, appended on the zhi edge of the
system box. This can be useful when a shock or wave is propagating
from zlo. This allows the system to grow with time to accommodate an
expanding wave. A simulation box must already exist, which is
typically created via the <a class="reference internal" href="create_box.html"><span class="doc">create_box</span></a> command.
Before using this command, a lattice must also be defined using the
<a class="reference internal" href="lattice.html"><span class="doc">lattice</span></a> command.</p>
<p>This fix will automatically freeze atoms on the zhi edge of the
system, so that overlaps are avoided when new atoms are appended.</p>
<p>The <em>basis</em> keyword specifies an atom type that will be assigned to
specific basis atoms as they are created. See the
<a class="reference internal" href="lattice.html"><span class="doc">lattice</span></a> command for specifics on how basis atoms are
defined for the unit cell of the lattice. By default, all created
atoms are assigned type = 1 unless this keyword specifies differently.</p>
<p>The <em>size</em> keyword defines the size in z of the chunk of material to
be added.</p>
<p>The <em>random</em> keyword will give the atoms random displacements around
their lattice points to simulate some initial temperature.</p>
<p>The <em>temp</em> keyword will cause a region to be thermostated with a
Langevin thermostat on the zhi boundary. The size of the region is
measured from zhi and is set with the <em>extent</em> argument.</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.
A <em>box</em> value selects standard distance units as defined by the
<span class="xref doc">units</span> command, e.g. Angstroms for units = real or metal.
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 <a class="reference internal" href="Section_howto.html#howto-15"><span class="std std-ref">output commands</span></a>. 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 boundary on which atoms are added with append/atoms must be
shrink/minimum. The opposite boundary may be any boundary type other
than periodic.</p>
</div>
<div class="section" id="related-commands">
<h2>Related commands</h2>
<p><a class="reference internal" href="fix_wall_piston.html"><span class="doc">fix wall/piston</span></a> command</p>
</div>
<div class="section" id="default">
<h2>Default</h2>
<p>The keyword defaults are size = 0.0, freq = 0, units = lattice. All
added atoms are of type 1 unless the basis keyword is used.</p>
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