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<title>bond_style quartic command &mdash; LAMMPS 15 May 2015 version documentation</title>
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<div class="section" id="bond-style-quartic-command">
<span id="index-0"></span><h1>bond_style quartic command<a class="headerlink" href="#bond-style-quartic-command" title="Permalink to this headline"></a></h1>
</div>
<div class="section" id="bond-style-quartic-omp-command">
<h1>bond_style quartic/omp command<a class="headerlink" href="#bond-style-quartic-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>bond_style quartic
</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>bond_style quartic
bond_coeff 2 1200 -0.55 0.25 1.3 34.6878
</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>quartic</em> bond style uses the potential</p>
<img alt="_images/bond_quartic.jpg" class="align-center" src="_images/bond_quartic.jpg" />
<p>to define a bond that can be broken as the simulation proceeds (e.g.
due to a polymer being stretched). The sigma and epsilon used in the
LJ portion of the formula are both set equal to 1.0 by LAMMPS.</p>
<p>The following coefficients must be defined for each bond type via the
<a class="reference internal" href="bond_coeff.html"><em>bond_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>K (energy/distance^4)</li>
<li>B1 (distance)</li>
<li>B2 (distance)</li>
<li>Rc (distance)</li>
<li>U0 (energy)</li>
</ul>
<p>This potential was constructed to mimic the FENE bond potential for
coarse-grained polymer chains. When monomers with sigma = epsilon =
1.0 are used, the following choice of parameters gives a quartic
potential that looks nearly like the FENE potential: K = 1200, B1 =
-0.55, B2 = 0.25, Rc = 1.3, and U0 = 34.6878. Different parameters
can be specified using the <a class="reference internal" href="bond_coeff.html"><em>bond_coeff</em></a> command, but
you will need to choose them carefully so they form a suitable bond
potential.</p>
<p>Rc is the cutoff length at which the bond potential goes smoothly to a
local maximum. If a bond length ever becomes &gt; Rc, LAMMPS &#8220;breaks&#8221;
the bond, which means two things. First, the bond potential is turned
off by setting its type to 0, and is no longer computed. Second, a
pairwise interaction between the two atoms is turned on, since they
are no longer bonded.</p>
<p>LAMMPS does the second task via a computational sleight-of-hand. It
subtracts the pairwise interaction as part of the bond computation.
When the bond breaks, the subtraction stops. For this to work, the
pairwise interaction must always be computed by the
<a class="reference internal" href="pair_style.html"><em>pair_style</em></a> command, whether the bond is broken or
not. This means that <a class="reference internal" href="special_bonds.html"><em>special_bonds</em></a> must be set
to 1,1,1, as indicated as a restriction below.</p>
<p>Note that when bonds are dumped to a file via the <a class="reference internal" href="dump.html"><em>dump local</em></a> command, bonds with type 0 are not included. The
<a class="reference internal" href="delete_bonds.html"><em>delete_bonds</em></a> command can also be used to query the
status of broken bonds or permanently delete them, e.g.:</p>
<div class="highlight-python"><div class="highlight"><pre>delete_bonds all stats
delete_bonds all bond 0 remove
</pre></div>
</div>
<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 bond style can only be used if LAMMPS was built with the
MOLECULE package (which it is by default). See the <a class="reference internal" href="Section_start.html#start-3"><span>Making LAMMPS</span></a> section for more info on packages.</p>
<p>The <em>quartic</em> style requires that <a class="reference internal" href="special_bonds.html"><em>special_bonds</em></a>
parameters be set to 1,1,1. Three- and four-body interactions (angle,
dihedral, etc) cannot be used with <em>quartic</em> bonds.</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="bond_coeff.html"><em>bond_coeff</em></a>, <a class="reference internal" href="delete_bonds.html"><em>delete_bonds</em></a></p>
<p><strong>Default:</strong> none</p>
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