"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c

:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)

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dihedral_style cosine/shift/exp command :h3
dihedral_style cosine/shift/exp/omp command :h3

[Syntax:]

dihedral_style cosine/shift/exp :pre

[Examples:]

dihedral_style cosine/shift/exp
dihedral_coeff 1 10.0 45.0 2.0 :pre

[Description:]

The {cosine/shift/exp} dihedral style uses the potential

:c,image(Eqs/dihedral_cosine_shift_exp.jpg)

where Umin, theta, and a are defined for each dihedral type.

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.

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.

The following coefficients must be defined for each dihedral type via
the "dihedral_coeff"_dihedral_coeff.html command as in the example
above, or in the data file or restart files read by the
"read_data"_read_data.html or "read_restart"_read_restart.html
commands:

umin (energy)
theta (angle)
A (real number) :ul

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Styles with a {cuda}, {gpu}, {omp}, or {opt} 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 "Section_accelerate"_Section_accelerate.html of the
manual.  The accelerated styles take the same arguments and should
produce the same results, except for round-off and precision issues.

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 "Making LAMMPS"_Section_start.html#start_3
section for more info.

You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the "-suffix command-line
switch"_Section_start.html#start_7 when you invoke LAMMPS, or you can
use the "suffix"_suffix.html command in your input script.

See "Section_accelerate"_Section_accelerate.html of the manual for
more instructions on how to use the accelerated styles effectively.

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[Restrictions:]

This dihedral style can only be used if LAMMPS was built with the
USER-MISC package.  See the "Making LAMMPS"_Section_start.html#start_3
section for more info on packages.

[Related commands:]

"dihedral_coeff"_dihedral_coeff.html,
"angle_cosineshiftexp"_angle_cosineshiftexp.html

[Default:] none
