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improper_class2.txt
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.. index:: improper_style class2
improper_style class2 command
=============================
improper_style class2/omp command
=================================
Syntax
""""""
.. parsed-literal::
improper_style class2
Examples
""""""""
.. parsed-literal::
improper_style class2
improper_coeff 1 100.0 0
improper_coeff * aa 0.0 0.0 0.0 115.06 130.01 115.06
Description
"""""""""""
The *class2* improper style uses the potential
.. image:: Eqs/improper_class2.jpg
:align: center
where Ei is the improper term and Eaa is an angle-angle term. The 3 X
terms in Ei are an average over 3 out-of-plane angles.
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. X_IJKL
refers to the angle between the plane of I,J,K and the plane of J,K,L,
and the bond JK lies in both planes. Similarly for X_KJLI and X_LJIK.
Note that atom J appears in the common bonds (JI, JK, JL) of all 3 X
terms. Thus J (the 2nd atom in the quadruplet) is the atom of
symmetry in the 3 X angles.
The subscripts on the various theta's refer to different combinations
of 3 atoms (I,J,K,L) used to form a particular angle. E.g. Theta_IJL
is the angle formed by atoms I,J,L with J in the middle. Theta1,
theta2, theta3 are the equilibrium positions of those angles. Again,
atom J (the 2nd atom in the quadruplet) is the atom of symmetry in the
theta angles, since it is always the center atom.
Since atom J is the atom of symmetry, normally the bonds J-I, J-K, J-L
would exist for an improper to be defined between the 4 atoms, but
this is not required.
See :ref:`(Sun) <Sun>` for a description of the COMPASS class2 force field.
Coefficients for the Ei and Eaa formulas 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.
These are the 2 coefficients for the Ei formula:
* 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.
For the Eaa formula, each line in a
:doc:`improper_coeff <improper_coeff>` command in the input script lists
7 coefficients, the first of which is "aa" to indicate they are
AngleAngle coefficients. In a data file, these coefficients should be
listed under a "AngleAngle Coeffs" heading and you must leave out the
"aa", i.e. only list 6 coefficients after the improper type.
* aa
* M1 (energy/distance)
* M2 (energy/distance)
* M3 (energy/distance)
* theta1 (degrees)
* theta2 (degrees)
* theta3 (degrees)
The theta values are specified in degrees, but LAMMPS converts them to
radians internally; hence the units of M 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
CLASS2 package. See the :ref:`Making LAMMPS <start_3>`
section for more info on packages.
Related commands
""""""""""""""""
:doc:`improper_coeff <improper_coeff>`
**Default:** none
----------
.. _Sun:
**(Sun)** Sun, J Phys Chem B 102, 7338-7364 (1998).
.. _lws: http://lammps.sandia.gov
.. _ld: Manual.html
.. _lc: Section_commands.html#comm
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