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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 helix command
</H3>
<H3>dihedral_style helix/omp command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>dihedral_style helix
</PRE>
<P><B>Examples:</B>
</P>
<PRE>dihedral_style helix
dihedral_coeff 1 80.0 100.0 40.0
</PRE>
<P><B>Description:</B>
</P>
<P>The <I>helix</I> dihedral style uses the potential
</P>
<CENTER><IMG SRC = "Eqs/dihedral_helix.jpg">
</CENTER>
<P>This coarse-grain dihedral potential is described in <A HREF = "#Guo">(Guo)</A>.
For dihedral angles in the helical region, the energy function is
represented by a standard potential consisting of three minima, one
corresponding to the trans (t) state and the other to gauche states
(g+ and g-). The paper describes how the A,B,C parameters are chosen
so as to balance secondary (largely driven by local interactions) and
tertiary structure (driven by long-range interactions).
</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>A (energy)
<LI>B (energy)
<LI>C (energy)
</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">Section_accelerate</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_7">-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">Section_accelerate</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
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><B>Related commands:</B>
</P>
<P><A HREF = "dihedral_coeff.html">dihedral_coeff</A>
</P>
<P><B>Default:</B> none
</P>
<HR>
<A NAME = "Guo"></A>
<P><B>(Guo)</B> Guo and Thirumalai, Journal of Molecular Biology, 263, 323-43 (1996).
</P>
</HTML>
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