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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 fene/expand command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>bond_style fene/expand
</PRE>
<P><B>Examples:</B>
</P>
<PRE>bond_style fene/expand
bond_coeff 1 30.0 1.5 1.0 1.0 0.5
</PRE>
<P><B>Description:</B>
</P>
<P>The <I>fene/expand</I> bond style uses the potential
</P>
<CENTER><IMG SRC = "Eqs/bond_fene_expand.jpg">
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<P>to define a finite extensible nonlinear elastic (FENE) potential
<A HREF = "#Kremer">(Kremer)</A>, used for bead-spring polymer models. The first
term is attractive, the 2nd Lennard-Jones term is repulsive.
</P>
<P>The <I>fene/expand</I> bond style is similar to <I>fene</I> except that an extra
shift factor of delta (positive or negative) is added to <I>r</I> to
effectively change the bead size of the bonded atoms. The first term
now extends to R0 + delta and the 2nd term is cutoff at 2^(1/6) sigma
+ delta.
</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>R0 (distance)
<LI>epsilon (energy)
<LI>sigma (distance)
<LI>delta (distance)
</UL>
<P><B>Restrictions:</B>
</P>
<P>Bond styles can only be set for atom styles that allow bonds to be
defined.
</P>
<P>This bond style is part of the "molecular" package. It is only
enabled if LAMMPS was built with that package. See the <A HREF = "Section_start.html#2_2">Making
LAMMPS</A> section for more info.
</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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<A NAME = "Kremer"></A>
<P><B>(Kremer)</B> Kremer, Grest, J Chem Phys, 92, 5057 (1990).
</P>
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