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improper_cossq.txt
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.. index:: improper_style cossq
improper_style cossq command
============================
improper_style cossq/omp command
================================
Syntax
""""""
.. parsed-literal::
improper_style cossq
Examples
""""""""
.. parsed-literal::
improper_style cossq
improper_coeff 1 4.0 0.0
Description
"""""""""""
The *cossq* improper style uses the potential
.. image:: Eqs/improper_cossq.jpg
:align: center
where x is the improper angle, x0 is its equilibrium value, and K is a
prefactor.
If the 4 atoms in an improper quadruplet (listed in the data file read
by the :doc:`read_data <read_data>` command) are ordered I,J,K,L then X
is the angle between the plane of I,J,K and the plane of J,K,L.
Alternatively, you can think of atoms J,K,L as being in a plane, and
atom I above the plane, and X as a measure of how far out-of-plane I
is with respect to the other 3 atoms.
Note that defining 4 atoms to interact in this way, does not mean that
bonds necessarily exist between I-J, J-K, or K-L, as they would in a
linear dihedral. Normally, the bonds I-J, I-K, I-L would exist for an
improper to be defined between the 4 atoms.
The following coefficients must be defined for each improper type via
the :doc:`improper_coeff <improper_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:
* K (energy/radian^2)
* X0 (degrees)
X0 is specified in degrees, but LAMMPS converts it to radians
internally; hence the units of K are in energy/radian^2.
----------
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 improper 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:`improper_coeff <improper_coeff>`
**Default:** none
.. _lws: http://lammps.sandia.gov
.. _ld: Manual.html
.. _lc: Section_commands.html#comm
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