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<div class="section" id="fix-momentum-command">
<span id="index-0"></span><h1>fix momentum command<a class="headerlink" href="#fix-momentum-command" title="Permalink to this headline"></a></h1>
<div class="section" id="syntax">
<h2>Syntax<a class="headerlink" href="#syntax" title="Permalink to this headline"></a></h2>
<div class="highlight-python"><div class="highlight"><pre>fix ID group-ID momentum N keyword values ...
</pre></div>
</div>
<ul class="simple">
<li>ID, group-ID are documented in <a class="reference internal" href="fix.html"><em>fix</em></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<a class="headerlink" href="#examples" title="Permalink to this headline"></a></h2>
<div class="highlight-python"><div class="highlight"><pre>fix 1 all momentum 1 linear 1 1 0
fix 1 all momentum 1 linear 1 1 1 rescale
fix 1 all momentum 100 linear 1 1 1 angular
</pre></div>
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<div class="section" id="description">
<h2>Description<a class="headerlink" href="#description" title="Permalink to this headline"></a></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"><em>fix langevin</em></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"><em>velocity</em></a> command can be used to create
initial velocities with zero aggregate linear and/or angular momentum.</p>
</div>
<div class="section" id="restart-fix-modify-output-run-start-stop-minimize-info">
<h2>Restart, fix_modify, output, run start/stop, minimize info<a class="headerlink" href="#restart-fix-modify-output-run-start-stop-minimize-info" title="Permalink to this headline"></a></h2>
<p>No information about this fix is written to <a class="reference internal" href="restart.html"><em>binary restart files</em></a>. None of the <a class="reference internal" href="fix_modify.html"><em>fix_modify</em></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>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"><em>run</em></a> command.
This fix is not invoked during <a class="reference internal" href="minimize.html"><em>energy minimization</em></a>.</p>
</div>
<div class="section" id="restrictions">
<h2>Restrictions<a class="headerlink" href="#restrictions" title="Permalink to this headline"></a></h2>
<blockquote>
<div>none</div></blockquote>
</div>
<div class="section" id="related-commands">
<h2>Related commands<a class="headerlink" href="#related-commands" title="Permalink to this headline"></a></h2>
<p><a class="reference internal" href="fix_recenter.html"><em>fix recenter</em></a>, <a class="reference internal" href="velocity.html"><em>velocity</em></a></p>
<p><strong>Default:</strong> none</p>
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