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<li class="toctree-l1"><a class="reference internal" href="Section_intro.html">1. Introduction</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_packages.html">4. Packages</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_accelerate.html">5. Accelerating LAMMPS performance</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_howto.html">6. How-to discussions</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_errors.html">12. Errors</a></li>
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<li>pair_style soft command</li>
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<div class="section" id="pair-style-soft-command">
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<span id="index-0"></span><h1>pair_style soft command</h1>
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</div>
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<div class="section" id="pair-style-soft-gpu-command">
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<h1>pair_style soft/gpu command</h1>
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</div>
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<div class="section" id="pair-style-soft-omp-command">
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<h1>pair_style soft/omp 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">pair_style</span> <span class="n">soft</span> <span class="n">cutoff</span>
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</pre></div>
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</div>
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<ul class="simple">
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<li>cutoff = global cutoff for soft interactions (distance units)</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">pair_style</span> <span class="n">soft</span> <span class="mf">1.0</span>
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<span class="n">pair_coeff</span> <span class="o">*</span> <span class="o">*</span> <span class="mf">10.0</span>
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<span class="n">pair_coeff</span> <span class="mi">1</span> <span class="mi">1</span> <span class="mf">10.0</span> <span class="mf">3.0</span>
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</pre></div>
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</div>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">pair_style</span> <span class="n">soft</span> <span class="mf">1.0</span>
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<span class="n">pair_coeff</span> <span class="o">*</span> <span class="o">*</span> <span class="mf">0.0</span>
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<span class="n">variable</span> <span class="n">prefactor</span> <span class="n">equal</span> <span class="n">ramp</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span><span class="mi">30</span><span class="p">)</span>
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<span class="n">fix</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">adapt</span> <span class="mi">1</span> <span class="n">pair</span> <span class="n">soft</span> <span class="n">a</span> <span class="o">*</span> <span class="o">*</span> <span class="n">v_prefactor</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>Style <em>soft</em> computes pairwise interactions with the formula</p>
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<img alt="_images/pair_soft.jpg" class="align-center" src="_images/pair_soft.jpg" />
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<p>It is useful for pushing apart overlapping atoms, since it does not
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blow up as r goes to 0. A is a pre-factor that can be made to vary in
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time from the start to the end of the run (see discussion below),
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e.g. to start with a very soft potential and slowly harden the
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interactions over time. Rc is the cutoff. See the <a class="reference internal" href="fix_nve_limit.html"><span class="doc">fix nve/limit</span></a> command for another way to push apart
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overlapping atoms.</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"><span class="doc">pair_coeff</span></a> command as in the examples 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"><span class="doc">read_data</span></a> or <a class="reference internal" href="read_restart.html"><span class="doc">read_restart</span></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>A (energy units)</li>
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<li>cutoff (distance units)</li>
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</ul>
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<p>The last coefficient is optional. If not specified, the global soft
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cutoff is used.</p>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">The syntax for <a class="reference internal" href="pair_coeff.html"><span class="doc">pair_coeff</span></a> with a single A
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coeff is different in the current version of LAMMPS than in older
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versions which took two values, Astart and Astop, to ramp between
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them. This functionality is now available in a more general form
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through the <a class="reference internal" href="fix_adapt.html"><span class="doc">fix adapt</span></a> command, as explained below.
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Note that if you use an old input script and specify Astart and Astop
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without a cutoff, then LAMMPS will interpret that as A and a cutoff,
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which is probabably not what you want.</p>
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</div>
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<p>The <a class="reference internal" href="fix_adapt.html"><span class="doc">fix adapt</span></a> command can be used to vary A for one
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or more pair types over the course of a simulation, in which case
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pair_coeff settings for A must still be specified, but will be
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overridden. For example these commands will vary the prefactor A for
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all pairwise interactions from 0.0 at the beginning to 30.0 at the end
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of a run:</p>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">variable</span> <span class="n">prefactor</span> <span class="n">equal</span> <span class="n">ramp</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span><span class="mi">30</span><span class="p">)</span>
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<span class="n">fix</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">adapt</span> <span class="mi">1</span> <span class="n">pair</span> <span class="n">soft</span> <span class="n">a</span> <span class="o">*</span> <span class="o">*</span> <span class="n">v_prefactor</span>
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</pre></div>
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</div>
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<p>Note that a formula defined by an <a class="reference internal" href="variable.html"><span class="doc">equal-style variable</span></a>
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can use the current timestep, elapsed time in the current run, elapsed
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time since the beginning of a series of runs, as well as access other
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variables.</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"><span class="doc">Section_accelerate</span></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 class="std std-ref">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 class="std std-ref">-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"><span class="doc">suffix</span></a> command in your input script.</p>
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<p>See <a class="reference internal" href="Section_accelerate.html"><span class="doc">Section_accelerate</span></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 A coefficient and cutoff
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distance for this pair style can be mixed. A is always mixed via a
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<em>geometric</em> rule. The cutoff is mixed according to the pair_modify
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mix value. The default mix value is <em>geometric</em>. See the
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“pair_modify” command for details.</p>
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<p>This pair style does not support the <a class="reference internal" href="pair_modify.html"><span class="doc">pair_modify</span></a>
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shift option, since the pair interaction goes to 0.0 at the cutoff.</p>
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<p>The <a class="reference internal" href="pair_modify.html"><span class="doc">pair_modify</span></a> table and tail options are not
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relevant for this pair style.</p>
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<p>This pair style writes its 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
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to be specified in an input script that reads a restart file.</p>
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<p>This pair style can only be used via the <em>pair</em> keyword of the
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<a class="reference internal" href="run_style.html"><span class="doc">run_style respa</span></a> command. It does not support the
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<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</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</h2>
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<p><a class="reference internal" href="pair_coeff.html"><span class="doc">pair_coeff</span></a>, <a class="reference internal" href="fix_nve_limit.html"><span class="doc">fix nve/limit</span></a>, <a class="reference internal" href="fix_adapt.html"><span class="doc">fix adapt</span></a></p>
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<p><strong>Default:</strong> none</p>
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