<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
<aclass="reference internal"href="bond_coeff.html"><spanclass="doc">bond_coeff</span></a> command as in the example above, or in
the data file or restart files read by the <aclass="reference internal"href="read_data.html"><spanclass="doc">read_data</span></a>
or <aclass="reference internal"href="read_restart.html"><spanclass="doc">read_restart</span></a> commands:</p>
<ulclass="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 <aclass="reference internal"href="bond_coeff.html"><spanclass="doc">bond_coeff</span></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 > Rc, LAMMPS “breaks”
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
<aclass="reference internal"href="pair_style.html"><spanclass="doc">pair_style</span></a> command, whether the bond is broken or
not. This means that <aclass="reference internal"href="special_bonds.html"><spanclass="doc">special_bonds</span></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 <aclass="reference internal"href="dump.html"><spanclass="doc">dump local</span></a> command, bonds with type 0 are not included. The
<aclass="reference internal"href="delete_bonds.html"><spanclass="doc">delete_bonds</span></a> command can also be used to query the
status of broken bonds or permanently delete them, e.g.:</p>
<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 <aclass="reference internal"href="Section_accelerate.html"><spanclass="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 <aclass="reference internal"href="Section_start.html#start-3"><spanclass="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 <aclass="reference internal"href="Section_start.html#start-7"><spanclass="std std-ref">-suffix command-line switch</span></a> when you invoke LAMMPS, or you can
use the <aclass="reference internal"href="suffix.html"><spanclass="doc">suffix</span></a> command in your input script.</p>
<p>See <aclass="reference internal"href="Section_accelerate.html"><spanclass="doc">Section_accelerate</span></a> of the manual for
more instructions on how to use the accelerated styles effectively.</p>
</div>
<hrclass="docutils"/>
<divclass="section"id="restrictions">
<h2>Restrictions</h2>
<p>This bond style can only be used if LAMMPS was built with the
MOLECULE package (which it is by default). See the <aclass="reference internal"href="Section_start.html#start-3"><spanclass="std std-ref">Making LAMMPS</span></a> section for more info on packages.</p>
<p>The <em>quartic</em> style requires that <aclass="reference internal"href="special_bonds.html"><spanclass="doc">special_bonds</span></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>
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