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<title>pair_style lj/cubic command — LAMMPS 15 May 2015 version documentation</title>
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<div class="section" id="pair-style-lj-cubic-command">
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<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>
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<div class="section" id="pair-style-lj-cubic-gpu-command">
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<h1>pair_style lj/cubic/gpu command<a class="headerlink" href="#pair-style-lj-cubic-gpu-command" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="pair-style-lj-cubic-omp-command">
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<h1>pair_style lj/cubic/omp command<a class="headerlink" href="#pair-style-lj-cubic-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>pair_style lj/cubic
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</pre></div>
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</div>
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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>pair_style lj/cubic
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pair_coeff * * 1.0 0.8908987
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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>The <em>lj/cubic</em> style computes a truncated LJ interaction potential
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whose energy and force are continuous everywhere. Inside the
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inflection point the interaction is identical to the standard 12/6
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<a class="reference internal" href="pair_lj.html"><em>Lennard-Jones</em></a> potential. The LJ function outside the
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inflection point is replaced with a cubic function of distance. The
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energy, force, and second derivative are continuous at the inflection
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point. The cubic coefficient A3 is chosen so that both energy and
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force go to zero at the cutoff distance. Outside the cutoff distance
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the energy and force are zero.</p>
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<img alt="_images/pair_lj_cubic.jpg" class="align-center" src="_images/pair_lj_cubic.jpg" />
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<p>The location of the inflection point rs is defined
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by the LJ diameter, rs/sigma = (26/7)^1/6. The cutoff distance
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is defined by rc/rs = 67/48 or rc/sigma = 1.737....
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The analytic expression for the
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the cubic coefficient
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A3*rmin^3/epsilon = 27.93... is given in the paper by
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Holian and Ravelo <a class="reference internal" href="#holian"><span>(Holian)</span></a>.</p>
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<p>This potential is commonly used to study the shock mechanics of FCC
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solids, as in Ravelo et al. <a class="reference internal" href="#ravelo"><span>(Ravelo)</span></a>.</p>
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<p>The following coefficients must be defined for each pair of atom types
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via the <a class="reference internal" href="pair_coeff.html"><em>pair_coeff</em></a> command as in the example above,
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or in the data file or restart files read by the
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<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>
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commands, or by mixing as described below:</p>
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<ul class="simple">
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<li>epsilon (energy units)</li>
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<li>sigma (distance units)</li>
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</ul>
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<p>Note that sigma is defined in the LJ formula as the zero-crossing
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distance for the potential, not as the energy minimum, which is
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located at rmin = 2^(1/6)*sigma. In the above example, sigma =
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0.8908987, so rmin = 1.</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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<hr class="docutils" />
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<p><strong>Mixing, shift, table, tail correction, restart, rRESPA info</strong>:</p>
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<p>For atom type pairs I,J and I != J, the epsilon and sigma coefficients
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and cutoff distance for all of the lj/cut pair styles can be mixed.
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The default mix value is <em>geometric</em>. See the “pair_modify” command
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for details.</p>
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<p>The lj/cubic pair style does not support the
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<a class="reference internal" href="pair_modify.html"><em>pair_modify</em></a> shift option,
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since pair interaction is already smoothed to 0.0 at the
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cutoff.</p>
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<p>The <a class="reference internal" href="pair_modify.html"><em>pair_modify</em></a> table option is not relevant
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for this pair style.</p>
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<p>The lj/cubic pair style does not support the
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<a class="reference internal" href="pair_modify.html"><em>pair_modify</em></a> tail option for adding long-range tail
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corrections to energy and pressure, since there are no corrections for
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a potential that goes to 0.0 at the cutoff.</p>
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<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
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not need to be specified in an input script that reads a restart file.</p>
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<p>The lj/cubic pair style can only be used via the <em>pair</em>
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keyword of the <a class="reference internal" href="run_style.html"><em>run_style respa</em></a> command. It does not
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support the <em>inner</em>, <em>middle</em>, <em>outer</em> keywords.</p>
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</div>
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<hr class="docutils" />
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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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<blockquote>
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<div>none</div></blockquote>
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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_coeff.html"><em>pair_coeff</em></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="ravelo"><span id="holian"></span><strong>(Holian)</strong> Holian and Ravelo, Phys Rev B, 51, 11275 (1995).</p>
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<p><strong>(Ravelo)</strong> Ravelo, Holian, Germann and Lomdahl, Phys Rev B, 70, 014103 (2004).</p>
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