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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>
<H3>dihedral_style cosine/shift/exp/omp 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>
<HR>
<P>Styles with a <I>cuda</I>, <I>gpu</I>, <I>omp</I>, or <I>opt</I> 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 HREF = "Section_accelerate.html">this section</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-OMP and OPT
packages, respectively. They are only enabled if LAMMPS was built with
those packages. See the <A HREF = "Section_start.html#start_3">Making LAMMPS</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 HREF = "Section_start.html#start_6">-suffix command-line
switch</A> when you invoke LAMMPS, or you can
use the <A HREF = "suffix.html">suffix</A> command in your input script.
</P>
<P>See <A HREF = "Section_accelerate.html">this section</A> of the manual for more
instructions on how to use the accelerated styles effectively.
</P>
<HR>
<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#start_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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