forked from lijiext/lammps
92 lines
3.2 KiB
HTML
92 lines
3.2 KiB
HTML
<HTML>
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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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</CENTER>
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<HR>
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<H3>dihedral_style cosine/shift/exp command
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</H3>
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<H3>dihedral_style cosine/shift/exp/omp command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>dihedral_style cosine/shift/exp
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</PRE>
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<P><B>Examples:</B>
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</P>
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<PRE>dihedral_style cosine/shift/exp
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dihedral_coeff 1 10.0 45.0 2.0
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>The <I>cosine/shift/exp</I> dihedral style uses the potential
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</P>
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<CENTER><IMG SRC = "Eqs/dihedral_cosine_shift_exp.jpg">
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</CENTER>
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<P>where Umin, theta, and a are defined for each dihedral type.
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</P>
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<P>The potential is bounded between [-Umin:0] and the minimum is located
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at the angle theta0. The a parameter can be both positive or negative
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and is used to control the spring constant at the equilibrium.
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</P>
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<P>The spring constant is given by k=a exp(a) Umin/ [2 (Exp(a)-1)].
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For a>3 k/Umin = a/2 to better than 5% relative error. For negative
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values of the a parameter, the spring constant is essentially zero,
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and anharmonic terms takes over. The potential is furthermore well
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behaved in the limit a->0, where it has been implemented to linear
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order in a for a < 0.001.
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</P>
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<P>The following coefficients must be defined for each dihedral type via
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the <A HREF = "dihedral_coeff.html">dihedral_coeff</A> command as in the example
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above, or in the data file or restart files read by the
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<A HREF = "read_data.html">read_data</A> or <A HREF = "read_restart.html">read_restart</A>
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commands:
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</P>
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<UL><LI>umin (energy)
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<LI>theta (angle)
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<LI>A (real number)
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</UL>
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<HR>
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<P>Styles with a <I>cuda</I>, <I>gpu</I>, <I>omp</I>, or <I>opt</I> suffix are functionally
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the same as the corresponding style without the suffix. They have
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been optimized to run faster, depending on your available hardware, as
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discussed in <A HREF = "Section_accelerate.html">Section_accelerate</A> of the
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manual. The accelerated styles take the same arguments and should
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produce the same results, except for round-off and precision issues.
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</P>
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<P>These accelerated styles are part of the USER-CUDA, GPU, USER-OMP and OPT
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packages, respectively. They are only enabled if LAMMPS was built with
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those packages. See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A>
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section for more info.
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</P>
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<P>You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the <A HREF = "Section_start.html#start_7">-suffix command-line
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switch</A> when you invoke LAMMPS, or you can
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use the <A HREF = "suffix.html">suffix</A> command in your input script.
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</P>
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<P>See <A HREF = "Section_accelerate.html">Section_accelerate</A> of the manual for
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more instructions on how to use the accelerated styles effectively.
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</P>
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<HR>
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<P><B>Restrictions:</B>
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</P>
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<P>This dihedral style can only be used if LAMMPS was built with the
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USER-MISC package. See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A>
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section for more info on packages.
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "dihedral_coeff.html">dihedral_coeff</A>,
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<A HREF = "angle_cosineshiftexp.html">angle_cosineshiftexp</A>
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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