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<div class="section" id="fix-momentum-command">
<span id="index-0"></span><h1>fix momentum 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">momentum</span> <span class="n">N</span> <span class="n">keyword</span> <span class="n">values</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>momentum = style name of this fix command</li>
<li>N = adjust the momentum every this many timesteps
one or more keyword/value pairs may be appended</li>
<li>keyword = <em>linear</em> or <em>angular</em> or <em>rescale</em></li>
</ul>
<pre class="literal-block">
<em>linear</em> values = xflag yflag zflag
xflag,yflag,zflag = 0/1 to exclude/include each dimension
<em>angular</em> values = none
</pre>
<pre class="literal-block">
<em>rescale</em> values = none
</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">momentum</span> <span class="mi">1</span> <span class="n">linear</span> <span class="mi">1</span> <span class="mi">1</span> <span class="mi">0</span>
<span class="n">fix</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">momentum</span> <span class="mi">1</span> <span class="n">linear</span> <span class="mi">1</span> <span class="mi">1</span> <span class="mi">1</span> <span class="n">rescale</span>
<span class="n">fix</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">momentum</span> <span class="mi">100</span> <span class="n">linear</span> <span class="mi">1</span> <span class="mi">1</span> <span class="mi">1</span> <span class="n">angular</span>
</pre></div>
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<div class="section" id="description">
<h2>Description</h2>
<p>Zero the linear and/or angular momentum of the group of atoms every N
timesteps by adjusting the velocities of the atoms. One (or both) of
the <em>linear</em> or <em>angular</em> keywords must be specified.</p>
<p>If the <em>linear</em> keyword is used, the linear momentum is zeroed by
subtracting the center-of-mass velocity of the group from each atom.
This does not change the relative velocity of any pair of atoms. One
or more dimensions can be excluded from this operation by setting the
corresponding flag to 0.</p>
<p>If the <em>angular</em> keyword is used, the angular momentum is zeroed by
subtracting a rotational component from each atom.</p>
<p>This command can be used to insure the entire collection of atoms (or
a subset of them) does not drift or rotate during the simulation due
to random perturbations (e.g. <a class="reference internal" href="fix_langevin.html"><span class="doc">fix langevin</span></a>
thermostatting).</p>
<p>The <em>rescale</em> keyword enables conserving the kinetic energy of the group
of atoms by rescaling the velocities after the momentum was removed.</p>
<p>Note that the <a class="reference internal" href="velocity.html"><span class="doc">velocity</span></a> command can be used to create
initial velocities with zero aggregate linear and/or angular momentum.</p>
<p><strong>Restart, fix_modify, output, run start/stop, minimize info:</strong></p>
<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>
<blockquote>
<div>none</div></blockquote>
</div>
<div class="section" id="related-commands">
<h2>Related commands</h2>
<p><a class="reference internal" href="fix_recenter.html"><span class="doc">fix recenter</span></a>, <a class="reference internal" href="velocity.html"><span class="doc">velocity</span></a></p>
<p><strong>Default:</strong> none</p>
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