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<li>fix qeq/reax command</li>
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<div class="section" id="fix-qeq-reax-command">
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<span id="index-0"></span><h1>fix qeq/reax 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">qeq</span><span class="o">/</span><span class="n">reax</span> <span class="n">Nevery</span> <span class="n">cutlo</span> <span class="n">cuthi</span> <span class="n">tolerance</span> <span class="n">params</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>qeq/reax = style name of this fix command</li>
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<li>Nevery = perform QEq every this many steps</li>
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<li>cutlo,cuthi = lo and hi cutoff for Taper radius</li>
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<li>tolerance = precision to which charges will be equilibrated</li>
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<li>params = reax/c or a filename</li>
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</ul>
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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">qeq</span><span class="o">/</span><span class="n">reax</span> <span class="mi">1</span> <span class="mf">0.0</span> <span class="mf">10.0</span> <span class="mf">1.0e-6</span> <span class="n">reax</span><span class="o">/</span><span class="n">c</span>
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<span class="n">fix</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">qeq</span><span class="o">/</span><span class="n">reax</span> <span class="mi">1</span> <span class="mf">0.0</span> <span class="mf">10.0</span> <span class="mf">1.0e-6</span> <span class="n">param</span><span class="o">.</span><span class="n">qeq</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>Perform the charge equilibration (QEq) method as described in <a class="reference internal" href="#rappe"><span class="std std-ref">(Rappe and Goddard)</span></a> and formulated in <a class="reference internal" href="neb.html#nakano"><span class="std std-ref">(Nakano)</span></a>. It is
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typically used in conjunction with the ReaxFF force field model as
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implemented in the <a class="reference internal" href="pair_reax_c.html"><span class="doc">pair_style reax/c</span></a> command, but
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it can be used with any potential in LAMMPS, so long as it defines and
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uses charges on each atom. The <a class="reference internal" href="fix_qeq_comb.html"><span class="doc">fix qeq/comb</span></a>
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command should be used to perform charge equliibration with the <a class="reference internal" href="pair_comb.html"><span class="doc">COMB potential</span></a>. For more technical details about the
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charge equilibration performed by fix qeq/reax, see the
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<a class="reference internal" href="pair_reax_c.html#aktulga"><span class="std std-ref">(Aktulga)</span></a> paper.</p>
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<p>The QEq method minimizes the electrostatic energy of the system by
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adjusting the partial charge on individual atoms based on interactions
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with their neighbors. It reqires some parameters for each atom type.
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If the <em>params</em> setting above is the word “reax/c”, then these are
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extracted from the <a class="reference internal" href="pair_reax_c.html"><span class="doc">pair_style reax/c</span></a> command and
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the ReaxFF force field file it reads in. If a file name is specified
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for <em>params</em>, then the parameters are taken from the specified file
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and the file must contain one line for each atom type. The latter
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form must be used when performing QeQ with a non-ReaxFF potential.
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Each line should be formatted as follows:</p>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">itype</span> <span class="n">chi</span> <span class="n">eta</span> <span class="n">gamma</span>
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</pre></div>
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</div>
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<p>where <em>itype</em> is the atom type from 1 to Ntypes, <em>chi</em> denotes the
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electronegativity in eV, <em>eta</em> denotes the self-Coulomb
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potential in eV, and <em>gamma</em> denotes the valence orbital
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exponent. Note that these 3 quantities are also in the ReaxFF
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potential file, except that eta is defined here as twice the eta value
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in the ReaxFF file. Note that unlike the rest of LAMMPS, the units
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of this fix are hard-coded to be A, eV, and electronic charge.</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>. No global scalar or vector or per-atom
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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
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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.</p>
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<p>This fix is 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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<p>This fix is part of the USER-REAXC package. It is only enabled if
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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>
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<p>This fix does not correctly handle interactions
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involving multiple periodic images of the same atom. Hence, it should not
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be used for periodic cell dimensions less than 10 angstroms.</p>
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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="pair_reax_c.html"><span class="doc">pair_style reax/c</span></a></p>
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<p><strong>Default:</strong> none</p>
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<hr class="docutils" />
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<p id="rappe"><strong>(Rappe)</strong> Rappe and Goddard III, Journal of Physical Chemistry, 95,
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3358-3363 (1991).</p>
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<p id="nakano"><strong>(Nakano)</strong> Nakano, Computer Physics Communications, 104, 59-69 (1997).</p>
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<p id="aktulga">(Aktulga) Aktulga, Fogarty, Pandit, Grama, Parallel Computing, 38,
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245-259 (2012).</p>
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</div>
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