92 lines
2.8 KiB
ReStructuredText
92 lines
2.8 KiB
ReStructuredText
.. index:: improper_style cossq
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improper_style cossq command
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============================
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improper_style cossq/omp command
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================================
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Syntax
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""""""
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.. code-block:: LAMMPS
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improper_style cossq
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Examples
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""""""""
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.. code-block:: LAMMPS
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improper_style cossq
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improper_coeff 1 4.0 0.0
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Description
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"""""""""""
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The *cossq* improper style uses the potential
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.. math::
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E = \frac{1}{2} K \cos^2{\left(\chi - \chi_0\right)}
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where :math:`\chi` is the improper angle, :math:`\chi_0` is its
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equilibrium value, and :math:`K` is a prefactor.
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If the 4 atoms in an improper quadruplet (listed in the data file read
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by the :doc:`read_data <read_data>` command) are ordered I,J,K,L then
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:math:`\chi` is the angle between the plane of I,J,K and the plane of J,K,L.
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Alternatively, you can think of atoms J,K,L as being in a plane, and
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atom I above the plane, and :math:`\chi` as a measure of how far
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out-of-plane I is with respect to the other 3 atoms.
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Note that defining 4 atoms to interact in this way, does not mean that
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bonds necessarily exist between I-J, J-K, or K-L, as they would in a
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linear dihedral. Normally, the bonds I-J, I-K, I-L would exist for an
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improper to be defined between the 4 atoms.
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The following coefficients must be defined for each improper type via
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the :doc:`improper_coeff <improper_coeff>` 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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:doc:`read_data <read_data>` or :doc:`read_restart <read_restart>`
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commands:
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* :math:`K` (energy)
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* :math:`\chi_0` (degrees)
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----------
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Styles with a *gpu*\ , *intel*\ , *kk*\ , *omp*\ , or *opt* suffix are
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functionally the same as the corresponding style without the suffix.
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They have been optimized to run faster, depending on your available
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hardware, as discussed on the :doc:`Speed packages <Speed_packages>` doc
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page. 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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These accelerated styles are part of the GPU, USER-INTEL, KOKKOS,
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USER-OMP and OPT packages, respectively. They are only enabled if
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LAMMPS was built with those packages. See the :doc:`Build package <Build_package>` doc page for more info.
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You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the :doc:`-suffix command-line switch <Run_options>` when you invoke LAMMPS, or you can use the
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:doc:`suffix <suffix>` command in your input script.
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See the :doc:`Speed packages <Speed_packages>` doc page for more
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instructions on how to use the accelerated styles effectively.
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----------
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Restrictions
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""""""""""""
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This improper style can only be used if LAMMPS was built with the
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USER-MISC package. See the :doc:`Build package <Build_package>` doc
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page for more info.
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Related commands
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""""""""""""""""
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:doc:`improper_coeff <improper_coeff>`
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**Default:** none
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