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<title>dihedral_style cosine/shift/exp command &mdash; LAMMPS 15 May 2015 version documentation</title>
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<div class="section" id="dihedral-style-cosine-shift-exp-command">
<span id="index-0"></span><h1>dihedral_style cosine/shift/exp command<a class="headerlink" href="#dihedral-style-cosine-shift-exp-command" title="Permalink to this headline"></a></h1>
</div>
<div class="section" id="dihedral-style-cosine-shift-exp-omp-command">
<h1>dihedral_style cosine/shift/exp/omp command<a class="headerlink" href="#dihedral-style-cosine-shift-exp-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>dihedral_style cosine/shift/exp
</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>dihedral_style cosine/shift/exp
dihedral_coeff 1 10.0 45.0 2.0
</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>cosine/shift/exp</em> dihedral style uses the potential</p>
<img alt="_images/dihedral_cosine_shift_exp.jpg" class="align-center" src="_images/dihedral_cosine_shift_exp.jpg" />
<p>where Umin, theta, and a are defined for each dihedral type.</p>
<p>The potential is bounded between [-Umin:0] and the minimum is located
at the angle theta0. The a parameter can be both positive or negative
and is used to control the spring constant at the equilibrium.</p>
<p>The spring constant is given by k=a exp(a) Umin/ [2 (Exp(a)-1)].
For a&gt;3 k/Umin = a/2 to better than 5% relative error. For negative
values of the a parameter, the spring constant is essentially zero,
and anharmonic terms takes over. The potential is furthermore well
behaved in the limit a-&gt;0, where it has been implemented to linear
order in a for a &lt; 0.001.</p>
<p>The following coefficients must be defined for each dihedral type via
the <a class="reference internal" href="dihedral_coeff.html"><em>dihedral_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:</p>
<ul class="simple">
<li>umin (energy)</li>
<li>theta (angle)</li>
<li>A (real number)</li>
</ul>
<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>
</div>
<hr class="docutils" />
<div class="section" id="restrictions">
<h2>Restrictions<a class="headerlink" href="#restrictions" title="Permalink to this headline"></a></h2>
<p>This dihedral style can only be used if LAMMPS was built with the
USER-MISC package. See the <a class="reference internal" href="Section_start.html#start-3"><span>Making LAMMPS</span></a>
section for more info on packages.</p>
</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="dihedral_coeff.html"><em>dihedral_coeff</em></a>,
<code class="xref doc docutils literal"><span class="pre">angle_cosineshiftexp</span></code></p>
<p><strong>Default:</strong> none</p>
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