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