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<div class="section" id="pair-style-soft-command">
<span id="index-0"></span><h1>pair_style soft command</h1>
</div>
<div class="section" id="pair-style-soft-gpu-command">
<h1>pair_style soft/gpu command</h1>
</div>
<div class="section" id="pair-style-soft-omp-command">
<h1>pair_style soft/omp command</h1>
<div class="section" id="syntax">
<h2>Syntax</h2>
<pre class="literal-block">
pair_style soft cutoff
</pre>
<ul class="simple">
<li>cutoff = global cutoff for soft interactions (distance units)</li>
</ul>
</div>
<div class="section" id="examples">
<h2>Examples</h2>
<pre class="literal-block">
pair_style soft 1.0
pair_coeff * * 10.0
pair_coeff 1 1 10.0 3.0
</pre>
<pre class="literal-block">
pair_style soft 1.0
pair_coeff * * 0.0
variable prefactor equal ramp(0,30)
fix 1 all adapt 1 pair soft a * * v_prefactor
</pre>
</div>
<div class="section" id="description">
<h2>Description</h2>
<p>Style <em>soft</em> computes pairwise interactions with the formula</p>
<img alt="_images/pair_soft.jpg" class="align-center" src="_images/pair_soft.jpg" />
<p>It is useful for pushing apart overlapping atoms, since it does not
blow up as r goes to 0. A is a pre-factor that can be made to vary in
time from the start to the end of the run (see discussion below),
e.g. to start with a very soft potential and slowly harden the
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
overlapping atoms.</p>
<p>The following coefficients must be defined for each pair of atom 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>A (energy units)</li>
<li>cutoff (distance units)</li>
</ul>
<p>The last coefficient is optional. If not specified, the global soft
cutoff is used.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<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
coeff is different in the current version of LAMMPS than in older
versions which took two values, Astart and Astop, to ramp between
them. This functionality is now available in a more general form
through the <a class="reference internal" href="fix_adapt.html"><span class="doc">fix adapt</span></a> command, as explained below.
Note that if you use an old input script and specify Astart and Astop
without a cutoff, then LAMMPS will interpret that as A and a cutoff,
which is probabably not what you want.</p>
</div>
<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
or more pair types over the course of a simulation, in which case
pair_coeff settings for A must still be specified, but will be
overridden. For example these commands will vary the prefactor A for
all pairwise interactions from 0.0 at the beginning to 30.0 at the end
of a run:</p>
<pre class="literal-block">
variable prefactor equal ramp(0,30)
fix 1 all adapt 1 pair soft a * * v_prefactor
</pre>
<p>Note that a formula defined by an <a class="reference internal" href="variable.html"><span class="doc">equal-style variable</span></a>
can use the current timestep, elapsed time in the current run, elapsed
time since the beginning of a series of runs, as well as access other
variables.</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 A coefficient and cutoff
distance for this pair style can be mixed. A is always mixed via a
<em>geometric</em> rule. The cutoff is mixed according to the pair_modify
mix value. The default mix value is <em>geometric</em>. See the
&#8220;pair_modify&#8221; command for details.</p>
<p>This pair style does not support the <a class="reference internal" href="pair_modify.html"><span class="doc">pair_modify</span></a>
shift option, since the pair interaction goes 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 and tail options are not
relevant for this pair style.</p>
<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
to be specified in an input script that reads a restart file.</p>
<p>This pair style 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. 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</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>, <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>
<p><strong>Default:</strong> none</p>
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