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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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<H3>bond_style quartic command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>bond_style quartic
</PRE>
<P><B>Examples:</B>
</P>
<PRE>bond_style quartic
bond_coeff 2 1200 -0.55 0.25 1.3 34.6878
</PRE>
<P><B>Description:</B>
</P>
<P>The <I>quartic</I> bond style uses the potential
</P>
<CENTER><IMG SRC = "Eqs/bond_quartic.jpg">
</CENTER>
<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 HREF = "bond_coeff.html">bond_coeff</A> command as in the example above, or in
the data file or restart files read by the <A HREF = "read_data.html">read_data</A>
or <A HREF = "read_restart.html">read_restart</A> commands:
</P>
<UL><LI>K (energy/distance^2)
<LI>B1 (distance)
<LI>B2 (distance)
<LI>Rc (distance)
<LI>U0 (energy)
</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 HREF = "bond_coeff.html">bond_coeff</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
<A HREF = "pair_style.html">pair_style</A> command, whether the bond is broken or
not. This means that <A HREF = "special_bonds.html">special_bonds</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 HREF = "dump.html">dump
local</A> command, bonds with type 0 are not included. The
<A HREF = "delete_bonds.html">delete_bonds</A> command can also be used to query the
status of broken bonds or permanently delete them, e.g.:
</P>
<PRE>delete_bonds all stats
delete_bonds all bond 0 remove
</PRE>
<P><B>Restrictions:</B>
</P>
<P>This bond style can only be used if LAMMPS was built with the
MOLECULAR package (which it is by default). See the <A HREF = "Section_start.html#start_3">Making
LAMMPS</A> section for more info on packages.
</P>
<P>The <I>quartic</I> style requires that <A HREF = "special_bonds.html">special_bonds</A>
parameters be set to 1,1,1. Three- and four-body interactions (angle,
dihedral, etc) cannot be used with <I>quartic</I> bonds.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "bond_coeff.html">bond_coeff</A>, <A HREF = "delete_bonds.html">delete_bonds</A>
</P>
<P><B>Default:</B> none
</P>
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