79 lines
1.9 KiB
ReStructuredText
79 lines
1.9 KiB
ReStructuredText
.. index:: improper_style cossq
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.. index:: improper_style cossq/omp
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improper_style cossq command
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============================
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Accelerator Variants: *cossq/omp*
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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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.. include:: accel_styles.rst
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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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MOLECULE 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
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"""""""
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none
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