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<div class="section" id="fix-qeq-comb-command">
<span id="index-0"></span><h1>fix qeq/comb command</h1>
</div>
<div class="section" id="fix-qeq-comb-omp-command">
<h1>fix qeq/comb/omp 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">qeq</span><span class="o">/</span><span class="n">comb</span> <span class="n">Nevery</span> <span class="n">precision</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>qeq/comb = style name of this fix command</li>
<li>Nevery = perform charge equilibration every this many steps</li>
<li>precision = convergence criterion for charge equilibration</li>
<li>zero or more keyword/value pairs may be appended</li>
<li>keyword = <em>file</em></li>
</ul>
<pre class="literal-block">
<em>file</em> value = filename
filename = name of file to write QEQ equilibration info to
</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">surface</span> <span class="n">qeq</span><span class="o">/</span><span class="n">comb</span> <span class="mi">10</span> <span class="mf">0.0001</span>
</pre></div>
</div>
</div>
<div class="section" id="description">
<h2>Description</h2>
<p>Perform charge equilibration (QeQ) in conjunction with the COMB
(Charge-Optimized Many-Body) potential as described in
<a class="reference internal" href="#comb-1"><span class="std std-ref">(COMB_1)</span></a> and <a class="reference internal" href="#comb-2"><span class="std std-ref">(COMB_2)</span></a>. It performs the charge
equilibration portion of the calculation using the so-called QEq
method, whereby the charge on each atom is adjusted to minimize the
energy of the system. This fix can only be used with the COMB
potential; see the <a class="reference internal" href="fix_qeq_reax.html"><span class="doc">fix qeq/reax</span></a> command for a QeQ
calculation that can be used with any potential.</p>
<p>Only charges on the atoms in the specified group are equilibrated.
The fix relies on the pair style (COMB in this case) to calculate the
per-atom electronegativity (effective force on the charges). An
electronegativity equalization calculation (or QEq) is performed in an
interative fashion, which in parallel requires communication at each
iteration for processors to exchange charge information about nearby
atoms with each other. See <a class="reference internal" href="#rappe-and-goddard"><span class="std std-ref">Rappe_and_Goddard</span></a> and
<a class="reference internal" href="#rick-and-stuart"><span class="std std-ref">Rick_and_Stuart</span></a> for details.</p>
<p>During a run, charge equilibration is peformed every <em>Nevery</em> time
steps. Charge equilibration is also always enforced on the first step
of each run. The <em>precision</em> argument controls the tolerance for the
difference in electronegativity for all atoms during charge
equilibration. <em>Precision</em> is a trade-off between the cost of
performing charge equilibration (more iterations) and accuracy.</p>
<p>If the <em>file</em> keyword is used, then information about each
equilibration calculation is written to the specifed file.</p>
<hr class="docutils" />
<p>Styles with a <em>gpu</em>, <em>intel</em>, <em>kk</em>, <em>omp</em>, or <em>opt</em> suffix are
functionally the same as the corresponding style without the suffix.
They have been optimized to run faster, depending on your available
hardware, as discussed in <a class="reference internal" href="Section_accelerate.html"><span class="doc">Section_accelerate</span></a>
of the manual. The accelerated styles take the same arguments and
should produce the same results, except for round-off and precision
issues.</p>
<p>These accelerated styles are part of the GPU, USER-INTEL, KOKKOS,
USER-OMP and OPT packages, respectively. They are only enabled if
LAMMPS was built with those packages. 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>You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the <a class="reference internal" href="Section_start.html#start-7"><span class="std std-ref">-suffix command-line switch</span></a> when you invoke LAMMPS, or you can
use the <a class="reference internal" href="suffix.html"><span class="doc">suffix</span></a> command in your input script.</p>
<p>See <a class="reference internal" href="Section_accelerate.html"><span class="doc">Section_accelerate</span></a> of the manual for
more instructions on how to use the accelerated styles effectively.</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>.</p>
<p>The <a class="reference internal" href="fix_modify.html"><span class="doc">fix_modify</span></a> <em>respa</em> option is supported by this
fix. This allows to set at which level of the <a class="reference internal" href="run_style.html"><span class="doc">r-RESPA</span></a>
integrator the fix is performing charge equilibration. Default is
the outermost level.</p>
<p>This fix produces a per-atom vector which can be accessed by various
<a class="reference internal" href="Section_howto.html#howto-15"><span class="std std-ref">output commands</span></a>. The vector stores the
gradient of the charge on each atom. The per-atom values be accessed
on any timestep.</p>
<p>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.</p>
<p>This fix can be 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 command currently only supports <a class="reference internal" href="pair_comb.html"><span class="doc">pair style *comb*</span></a>.</p>
</div>
<div class="section" id="related-commands">
<h2>Related commands</h2>
<p><a class="reference internal" href="pair_comb.html"><span class="doc">pair_style comb</span></a></p>
</div>
<div class="section" id="default">
<h2>Default</h2>
<p>No file output is performed.</p>
<hr class="docutils" />
<p id="comb-1"><strong>(COMB_1)</strong> J. Yu, S. B. Sinnott, S. R. Phillpot, Phys Rev B, 75, 085311 (2007),</p>
<p id="comb-2"><strong>(COMB_2)</strong> T.-R. Shan, B. D. Devine, T. W. Kemper, S. B. Sinnott, S. R.
Phillpot, Phys Rev B, 81, 125328 (2010).</p>
<p id="rappe-and-goddard"><strong>(Rappe_and_Goddard)</strong> A. K. Rappe, W. A. Goddard, J Phys Chem 95, 3358
(1991).</p>
<p id="rick-and-stuart"><strong>(Rick_and_Stuart)</strong> S. W. Rick, S. J. Stuart, B. J. Berne, J Chem Phys
101, 16141 (1994).</p>
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