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<title>pair_style lj/cubic command &mdash; LAMMPS 15 May 2015 version documentation</title>
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<div class="section" id="pair-style-lj-cubic-command">
<span id="index-0"></span><h1>pair_style lj/cubic command<a class="headerlink" href="#pair-style-lj-cubic-command" title="Permalink to this headline"></a></h1>
</div>
<div class="section" id="pair-style-lj-cubic-omp-command">
<h1>pair_style lj/cubic/omp command<a class="headerlink" href="#pair-style-lj-cubic-omp-command" title="Permalink to this headline"></a></h1>
<div class="section" id="syntax">
<h2>Syntax<a class="headerlink" href="#syntax" title="Permalink to this headline"></a></h2>
<div class="highlight-python"><div class="highlight"><pre>pair_style lj/cubic
</pre></div>
</div>
</div>
<div class="section" id="examples">
<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline"></a></h2>
<div class="highlight-python"><div class="highlight"><pre>pair_style lj/cubic
pair_coeff * * 1.0 0.8908987
</pre></div>
</div>
</div>
<div class="section" id="description">
<h2>Description<a class="headerlink" href="#description" title="Permalink to this headline"></a></h2>
<p>The <em>lj/cubic</em> style computes a truncated LJ interaction potential whose
energy and force are continuous everywhere.
Inside the inflection point the interaction is identical to the
standard 12/6 <a class="reference internal" href="pair_lj.html"><em>Lennard-Jones</em></a> potential.
The LJ function outside the inflection point is replaced
with a cubic function of distance. The energy, force, and second
derivative are continuous at the inflection point.
The cubic coefficient A3 is chosen so
that both energy and force go to zero at the cutoff distance.
Outside the cutoff distance the energy and force are zero.</p>
<img alt="_images/pair_lj_cubic.jpg" class="align-center" src="_images/pair_lj_cubic.jpg" />
<p>The location of the inflection point rs is defined
by the LJ diameter, rs/sigma = (26/7)^1/6. The cutoff distance
is defined by rc/rs = 67/48 or rc/sigma = 1.737....
The analytic expression for the
the cubic coefficient
A3*rmin^3/epsilon = 27.93... is given in the paper by
Holian and Ravelo <a class="reference internal" href="#holian"><span>(Holian)</span></a>.</p>
<p>This potential is commonly used to study the shock mechanics
of FCC solids, as in Ravelo et al. <a class="reference internal" href="#ravelo"><span>(Ravelo)</span></a>.</p>
<p>The following coefficients must be defined for each pair of atom
types via the <a class="reference internal" href="pair_coeff.html"><em>pair_coeff</em></a> command as in the example
above, or in the data file or restart files read by the
<a class="reference internal" href="read_data.html"><em>read_data</em></a> or <a class="reference internal" href="read_restart.html"><em>read_restart</em></a>
commands, or by mixing as described below:</p>
<ul class="simple">
<li>epsilon (energy units)</li>
<li>sigma (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, which
is located at rmin = 2^(1/6)*sigma. In the above example, sigma = 0.8908987,
so rmin = 1.</p>
<hr class="docutils" />
<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
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"><em>Section_accelerate</em></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 USER-CUDA, 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>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>-suffix command-line switch</span></a> when you invoke LAMMPS, or you can
use the <a class="reference internal" href="suffix.html"><em>suffix</em></a> command in your input script.</p>
<p>See <a class="reference internal" href="Section_accelerate.html"><em>Section_accelerate</em></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>The lj/cubic pair style does not support the
<a class="reference internal" href="pair_modify.html"><em>pair_modify</em></a> shift option,
since pair interaction is already smoothed to 0.0 at the
cutoff.</p>
<p>The <a class="reference internal" href="pair_modify.html"><em>pair_modify</em></a> table option is not relevant
for this pair style.</p>
<p>The lj/cubic pair style does not support the
<a class="reference internal" href="pair_modify.html"><em>pair_modify</em></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>The lj/cubic pair style writes its information to <a class="reference internal" href="restart.html"><em>binary restart files</em></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>The lj/cubic pair style can only be used via the <em>pair</em>
keyword of the <a class="reference internal" href="run_style.html"><em>run_style respa</em></a> command. It does 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<a class="headerlink" href="#restrictions" title="Permalink to this headline"></a></h2>
<blockquote>
<div>none</div></blockquote>
</div>
<div class="section" id="related-commands">
<h2>Related commands<a class="headerlink" href="#related-commands" title="Permalink to this headline"></a></h2>
<p><a class="reference internal" href="pair_coeff.html"><em>pair_coeff</em></a></p>
<p><strong>Default:</strong> none</p>
<hr class="docutils" />
<p id="ravelo"><span id="holian"></span><strong>(Holian)</strong> Holian and Ravelo, Phys Rev B, 51, 11275 (1995).</p>
<p><strong>(Ravelo)</strong> Ravelo, Holian, Germann and Lomdahl, Phys Rev B, 70, 014103 (2004).</p>
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