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<title>fix qeq/comb command — LAMMPS 15 May 2015 version documentation</title>
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<div class="section" id="fix-qeq-comb-command">
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<span id="index-0"></span><h1>fix qeq/comb command<a class="headerlink" href="#fix-qeq-comb-command" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="fix-qeq-comb-omp-command">
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<h1>fix qeq/comb/omp command<a class="headerlink" href="#fix-qeq-comb-omp-command" title="Permalink to this headline">¶</a></h1>
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<div class="section" id="syntax">
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<h2>Syntax<a class="headerlink" href="#syntax" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>fix ID group-ID qeq/comb Nevery precision keyword value ...
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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"><em>fix</em></a> command</li>
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<li>qeq/comb = style name of this fix command</li>
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<li>Nevery = perform charge equilibration every this many steps</li>
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<li>precision = convergence criterion for charge equilibration</li>
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<li>zero or more keyword/value pairs may be appended</li>
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<li>keyword = <em>file</em></li>
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</ul>
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<pre class="literal-block">
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<em>file</em> value = filename
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filename = name of file to write QEQ equilibration info to
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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<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>fix 1 surface qeq/comb 10 0.0001
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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<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h2>
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<p>Perform charge equilibration (QeQ) in conjunction with the COMB
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(Charge-Optimized Many-Body) potential as described in
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<a class="reference internal" href="#comb-1"><span>(COMB_1)</span></a> and <a class="reference internal" href="#comb-2"><span>(COMB_2)</span></a>. It performs the charge
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equilibration portion of the calculation using the so-called QEq
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method, whereby the charge on each atom is adjusted to minimize the
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energy of the system. This fix can only be used with the COMB
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potential; see the <a class="reference internal" href="fix_qeq_reax.html"><em>fix qeq/reax</em></a> command for a QeQ
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calculation that can be used with any potential.</p>
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<p>Only charges on the atoms in the specified group are equilibrated.
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The fix relies on the pair style (COMB in this case) to calculate the
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per-atom electronegativity (effective force on the charges). An
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electronegativity equalization calculation (or QEq) is performed in an
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interative fashion, which in parallel requires communication at each
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iteration for processors to exchange charge information about nearby
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atoms with each other. See <a class="reference internal" href="#rappe-and-goddard"><span>Rappe_and_Goddard</span></a> and
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<a class="reference internal" href="#rick-and-stuart"><span>Rick_and_Stuart</span></a> for details.</p>
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<p>During a run, charge equilibration is peformed every <em>Nevery</em> time
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steps. Charge equilibration is also always enforced on the first step
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of each run. The <em>precision</em> argument controls the tolerance for the
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difference in electronegativity for all atoms during charge
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equilibration. <em>Precision</em> is a trade-off between the cost of
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performing charge equilibration (more iterations) and accuracy.</p>
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<p>If the <em>file</em> keyword is used, then information about each
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equilibration calculation is written to the specifed file.</p>
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<hr class="docutils" />
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<p>Styles with a <em>cuda</em>, <em>gpu</em>, <em>intel</em>, <em>kk</em>, <em>omp</em>, or <em>opt</em> suffix are
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functionally the same as the corresponding style without the suffix.
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They have been optimized to run faster, depending on your available
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hardware, as discussed in <a class="reference internal" href="Section_accelerate.html"><em>Section_accelerate</em></a>
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of the manual. The accelerated styles take the same arguments and
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should produce the same results, except for round-off and precision
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issues.</p>
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<p>These accelerated styles are part of the USER-CUDA, GPU, USER-INTEL,
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KOKKOS, USER-OMP and OPT packages, respectively. They are only
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enabled if LAMMPS was built with those packages. See the <a class="reference internal" href="Section_start.html#start-3"><span>Making LAMMPS</span></a> section for more info.</p>
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<p>You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the <a class="reference internal" href="Section_start.html#start-7"><span>-suffix command-line switch</span></a> when you invoke LAMMPS, or you can
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use the <a class="reference internal" href="suffix.html"><em>suffix</em></a> command in your input script.</p>
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<p>See <a class="reference internal" href="Section_accelerate.html"><em>Section_accelerate</em></a> of the manual for
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more instructions on how to use the accelerated styles effectively.</p>
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</div>
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<hr class="docutils" />
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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<a class="headerlink" href="#restart-fix-modify-output-run-start-stop-minimize-info" title="Permalink to this headline">¶</a></h2>
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<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
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are relevant to this fix.</p>
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<p>This fix produces a per-atom vector which can be accessed by various
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<a class="reference internal" href="Section_howto.html#howto-15"><span>output commands</span></a>. The vector stores the
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gradient of the charge on each atom. The per-atom values be accessed
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on any timestep.</p>
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<p>No parameter of this fix can be used with the <em>start/stop</em> keywords of
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the <a class="reference internal" href="run.html"><em>run</em></a> command.</p>
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<p>This fix can be invoked during <a class="reference internal" href="minimize.html"><em>energy minimization</em></a>.</p>
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</div>
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<div class="section" id="restrictions">
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<h2>Restrictions<a class="headerlink" href="#restrictions" title="Permalink to this headline">¶</a></h2>
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<p>This fix command currently only supports <a class="reference internal" href="pair_comb.html"><em>pair style *comb*</em></a>.</p>
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</div>
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<div class="section" id="related-commands">
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<h2>Related commands<a class="headerlink" href="#related-commands" title="Permalink to this headline">¶</a></h2>
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<p><a class="reference internal" href="pair_comb.html"><em>pair_style comb</em></a></p>
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</div>
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<div class="section" id="default">
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<h2>Default<a class="headerlink" href="#default" title="Permalink to this headline">¶</a></h2>
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<p>No file output is performed.</p>
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<hr class="docutils" />
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<p id="comb-1"><strong>(COMB_1)</strong> J. Yu, S. B. Sinnott, S. R. Phillpot, Phys Rev B, 75, 085311 (2007),</p>
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<p id="comb-2"><strong>(COMB_2)</strong> T.-R. Shan, B. D. Devine, T. W. Kemper, S. B. Sinnott, S. R.
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Phillpot, Phys Rev B, 81, 125328 (2010).</p>
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<p id="rappe-and-goddard"><strong>(Rappe_and_Goddard)</strong> A. K. Rappe, W. A. Goddard, J Phys Chem 95, 3358
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(1991).</p>
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<p id="rick-and-stuart"><strong>(Rick_and_Stuart)</strong> S. W. Rick, S. J. Stuart, B. J. Berne, J Chem Phys
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101, 16141 (1994).</p>
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