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lammps/doc/_sources/dihedral_cosine_shift_exp.txt
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dihedral_cosine_shift_exp.txt
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.. index:: dihedral_style cosine/shift/exp
dihedral_style cosine/shift/exp command
=======================================
dihedral_style cosine/shift/exp/omp command
===========================================
Syntax
""""""
.. parsed-literal::
dihedral_style cosine/shift/exp
Examples
""""""""
.. parsed-literal::
dihedral_style cosine/shift/exp
dihedral_coeff 1 10.0 45.0 2.0
Description
"""""""""""
The *cosine/shift/exp* dihedral style uses the potential
.. image:: Eqs/dihedral_cosine_shift_exp.jpg
:align: center
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 :doc:`dihedral_coeff <dihedral_coeff>` command as in the example
above, or in the data file or restart files read by the
:doc:`read_data <read_data>` or :doc:`read_restart <read_restart>`
commands:
* umin (energy)
* theta (angle)
* A (real number)
----------
Styles with a *cuda*, *gpu*, *intel*, *kk*, *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 :doc:`Section_accelerate <Section_accelerate>`
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-INTEL,
KOKKOS, USER-OMP and OPT packages, respectively. They are only
enabled if LAMMPS was built with those packages. See the :ref:`Making LAMMPS <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 :ref:`-suffix command-line switch <start_7>` when you invoke LAMMPS, or you can
use the :doc:`suffix <suffix>` command in your input script.
See :doc:`Section_accelerate <Section_accelerate>` of the manual for
more instructions on how to use the accelerated styles effectively.
----------
Restrictions
""""""""""""
This dihedral style can only be used if LAMMPS was built with the
USER-MISC package. See the :ref:`Making LAMMPS <start_3>`
section for more info on packages.
Related commands
""""""""""""""""
:doc:`dihedral_coeff <dihedral_coeff>`,
:doc:`angle_cosineshiftexp <angle_cosineshiftexp>`
**Default:** none
.. _lws: http://lammps.sandia.gov
.. _ld: Manual.html
.. _lc: Section_commands.html#comm
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