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<div class="section" id="fix-lb-momentum-command">
<span id="index-0"></span><h1>fix lb/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">lb</span><span class="o">/</span><span class="n">momentum</span> <span class="n">nevery</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 the <a class="reference internal" href="fix.html"><span class="doc">fix</span></a> command</li>
<li>lb/momentum = style name of this fix command</li>
<li>nevery = adjust the momentum every this many timesteps</li>
<li>zero or more keyword/value pairs may be appended</li>
<li>keyword = <em>linear</em></li>
</ul>
<pre class="literal-block">
<em>linear</em> values = xflag yflag zflag
xflag,yflag,zflag = 0/1 to exclude/include each dimension.
</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="n">sphere</span> <span class="n">lb</span><span class="o">/</span><span class="n">momentum</span>
<span class="n">fix</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">lb</span><span class="o">/</span><span class="n">momentum</span> <span class="n">linear</span> <span class="mi">1</span> <span class="mi">1</span> <span class="mi">0</span>
</pre></div>
</div>
</div>
<div class="section" id="description">
<h2>Description</h2>
<p>This fix is based on the <a class="reference internal" href="fix_momentum.html"><span class="doc">fix momentum</span></a> command, and
was created to be used in place of that command, when a
lattice-Boltzmann fluid is present.</p>
<p>Zero the total linear momentum of the system, including both the atoms
specified by group-ID and the lattice-Boltzmann fluid every nevery
timesteps. This is accomplished by adjusting the particle velocities
and the fluid velocities at each lattice site.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">This fix only considers the linear momentum of the system.</p>
</div>
<p>By default, the subtraction is performed for each dimension. This can
be changed by specifying the keyword <em>linear</em>, along with a set of
three flags set to 0/1 in order to exclude/ include the corresponding
dimension.</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>
<p>Can only be used if a lattice-Boltzmann fluid has been created via the
<a class="reference internal" href="fix_lb_fluid.html"><span class="doc">fix lb/fluid</span></a> command, and must come after this
command.</p>
<p>This fix is part of the USER-LB 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>
</div>
<div class="section" id="related-commands">
<h2>Related commands</h2>
<p><a class="reference internal" href="fix_momentum.html"><span class="doc">fix momentum</span></a>, <a class="reference internal" href="fix_lb_fluid.html"><span class="doc">fix lb/fluid</span></a></p>
</div>
<div class="section" id="default">
<h2>Default</h2>
<p>Zeros the total system linear momentum in each dimension.</p>
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