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<div class="section" id="pair-style-lj-gromacs-command">
<span id="index-0"></span><h1>pair_style lj/gromacs command</h1>
</div>
<div class="section" id="pair-style-lj-gromacs-gpu-command">
<h1>pair_style lj/gromacs/gpu command</h1>
</div>
<div class="section" id="pair-style-lj-gromacs-omp-command">
<h1>pair_style lj/gromacs/omp command</h1>
</div>
<div class="section" id="pair-style-lj-gromacs-coul-gromacs-command">
<h1>pair_style lj/gromacs/coul/gromacs command</h1>
</div>
<div class="section" id="pair-style-lj-gromacs-coul-gromacs-omp-command">
<h1>pair_style lj/gromacs/coul/gromacs/omp command</h1>
<div class="section" id="syntax">
<h2>Syntax</h2>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">pair_style</span> <span class="n">style</span> <span class="n">args</span>
</pre></div>
</div>
<ul class="simple">
<li>style = <em>lj/gromacs</em> or <em>lj/gromacs/coul/gromacs</em></li>
<li>args = list of arguments for a particular style</li>
</ul>
<pre class="literal-block">
<em>lj/gromacs</em> args = inner outer
inner, outer = global switching cutoffs for Lennard Jones
<em>lj/gromacs/coul/gromacs</em> args = inner outer (inner2) (outer2)
inner, outer = global switching cutoffs for Lennard Jones (and Coulombic if only 2 args)
inner2, outer2 = global switching cutoffs for Coulombic (optional)
</pre>
</div>
<div class="section" id="examples">
<h2>Examples</h2>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">pair_style</span> <span class="n">lj</span><span class="o">/</span><span class="n">gromacs</span> <span class="mf">9.0</span> <span class="mf">12.0</span>
<span class="n">pair_coeff</span> <span class="o">*</span> <span class="o">*</span> <span class="mf">100.0</span> <span class="mf">2.0</span>
<span class="n">pair_coeff</span> <span class="mi">2</span> <span class="mi">2</span> <span class="mf">100.0</span> <span class="mf">2.0</span> <span class="mf">8.0</span> <span class="mf">10.0</span>
</pre></div>
</div>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">pair_style</span> <span class="n">lj</span><span class="o">/</span><span class="n">gromacs</span><span class="o">/</span><span class="n">coul</span><span class="o">/</span><span class="n">gromacs</span> <span class="mf">9.0</span> <span class="mf">12.0</span>
<span class="n">pair_style</span> <span class="n">lj</span><span class="o">/</span><span class="n">gromacs</span><span class="o">/</span><span class="n">coul</span><span class="o">/</span><span class="n">gromacs</span> <span class="mf">8.0</span> <span class="mf">10.0</span> <span class="mf">7.0</span> <span class="mf">9.0</span>
<span class="n">pair_coeff</span> <span class="o">*</span> <span class="o">*</span> <span class="mf">100.0</span> <span class="mf">2.0</span>
</pre></div>
</div>
</div>
<div class="section" id="description">
<h2>Description</h2>
<p>The <em>lj/gromacs</em> styles compute shifted LJ and Coulombic interactions
with an additional switching function S(r) that ramps the energy and force
smoothly to zero between an inner and outer cutoff. It is a commonly
used potential in the <a class="reference external" href="http://www.gromacs.org">GROMACS</a> MD code and for
the coarse-grained models of <a class="reference internal" href="#marrink"><span class="std std-ref">(Marrink)</span></a>.</p>
<img alt="_images/pair_gromacs.jpg" class="align-center" src="_images/pair_gromacs.jpg" />
<p>r1 is the inner cutoff; rc is the outer cutoff. The coefficients A, B,
and C are computed by LAMMPS to perform the shifting and smoothing.
The function
S(r) is actually applied once to each term of the LJ formula and once
to the Coulombic formula, so there are 2 or 3 sets of A,B,C coefficients
depending on which pair_style is used. The boundary conditions
applied to the smoothing function are as follows: S&#8217;(r1) = S&#8217;&#8216;(r1) = 0,
S(rc) = -E(rc), S&#8217;(rc) = -E&#8217;(rc), and S&#8217;&#8216;(rc) = -E&#8217;&#8216;(rc),
where E(r) is the corresponding term
in the LJ or Coulombic potential energy function.
Single and double primes denote first and second
derivatives with respect to r, respectively.</p>
<p>The inner and outer cutoff for the LJ and Coulombic terms can be the
same or different depending on whether 2 or 4 arguments are used in
the pair_style command. The inner LJ cutoff must be &gt; 0, but the
inner Coulombic cutoff can be &gt;= 0.</p>
<p>The following coefficients must be defined for each pair of atoms
types via the <a class="reference internal" href="pair_coeff.html"><span class="doc">pair_coeff</span></a> command as in the examples
above, or in the data file or restart files read by the
<a class="reference internal" href="read_data.html"><span class="doc">read_data</span></a> or <a class="reference internal" href="read_restart.html"><span class="doc">read_restart</span></a>
commands, or by mixing as described below:</p>
<ul class="simple">
<li>epsilon (energy units)</li>
<li>sigma (distance units)</li>
<li>inner (distance units)</li>
<li>outer (distance units)</li>
</ul>
<p>Note that sigma is defined in the LJ formula as the zero-crossing
distance for the potential, not as the energy minimum at 2^(1/6)
sigma.</p>
<p>The last 2 coefficients are optional inner and outer cutoffs for style
<em>lj/gromacs</em>. If not specified, the global <em>inner</em> and <em>outer</em> values
are used.</p>
<p>The last 2 coefficients cannot be used with style
<em>lj/gromacs/coul/gromacs</em> because this force field does not allow
varying cutoffs for individual atom pairs; all pairs use the global
cutoff(s) specified in the pair_style command.</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>
<hr class="docutils" />
<p><strong>Mixing, shift, table, tail correction, restart, rRESPA info</strong>:</p>
<p>For atom type pairs I,J and I != J, the epsilon and sigma coefficients
and cutoff distance for all of the lj/cut pair styles can be mixed.
The default mix value is <em>geometric</em>. See the &#8220;pair_modify&#8221; command
for details.</p>
<p>None of the GROMACS pair styles support the
<a class="reference internal" href="pair_modify.html"><span class="doc">pair_modify</span></a> shift option, since the Lennard-Jones
portion of the pair interaction is already smoothed to 0.0 at the
cutoff.</p>
<p>The <a class="reference internal" href="pair_modify.html"><span class="doc">pair_modify</span></a> table option is not relevant
for this pair style.</p>
<p>None of the GROMACS pair styles support the
<a class="reference internal" href="pair_modify.html"><span class="doc">pair_modify</span></a> tail option for adding long-range tail
corrections to energy and pressure, since there are no corrections for
a potential that goes to 0.0 at the cutoff.</p>
<p>All of the GROMACS pair styles write their information to <a class="reference internal" href="restart.html"><span class="doc">binary restart files</span></a>, so pair_style and pair_coeff commands do
not need to be specified in an input script that reads a restart file.</p>
<p>All of the GROMACS pair styles can only be used via the <em>pair</em>
keyword of the <a class="reference internal" href="run_style.html"><span class="doc">run_style respa</span></a> command. They do not
support the <em>inner</em>, <em>middle</em>, <em>outer</em> keywords.</p>
</div>
<hr class="docutils" />
<div class="section" id="restrictions">
<h2>Restrictions</h2>
<blockquote>
<div>none</div></blockquote>
</div>
<div class="section" id="related-commands">
<h2>Related commands</h2>
<p><a class="reference internal" href="pair_coeff.html"><span class="doc">pair_coeff</span></a></p>
<p><strong>Default:</strong> none</p>
<hr class="docutils" />
<p id="marrink"><strong>(Marrink)</strong> Marrink, de Vries, Mark, J Phys Chem B, 108, 750-760 (2004).</p>
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