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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>
</CENTER>
<HR>
<H3>dihedral_style cosine/shift/exp command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>dihedral_style cosine/shift/exp
</PRE>
<P><B>Examples:</B>
</P>
<PRE>dihedral_style cosine/shift/exp
dihedral_coeff 1 10.0 45.0 2.0
</PRE>
<P><B>Description:</B>
</P>
<P>The <I>cosine/shift/exp</I> dihedral style uses the potential
</P>
<CENTER><IMG SRC = "Eqs/dihedral_cosine_shift_exp.jpg">
</CENTER>
<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>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->0, where it has been implemented to linear
order in a for a < 0.001.
</P>
<P>The following coefficients must be defined for each dihedral type via
the <A HREF = "dihedral_coeff.html">dihedral_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>umin (energy)
<LI>theta (angle)
<LI>A (real number)
</UL>
<P><B>Restrictions:</B>
</P>
<P>This dihedral style can only be used if LAMMPS was built with the
"user-misc" package. See the <A HREF = "Section_start.html#2_3">Making LAMMPS</A>
section for more info on packages.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "dihedral_coeff.html">dihedral_coeff</A>,
<A HREF = "angle_cosineshiftexp.html">angle_cosineshiftexp</A>
</P>
<P><B>Default:</B> none
</P>
</HTML>
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