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pair_class2.txt
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.. index:: pair_style lj/class2
pair_style lj/class2 command
============================
pair_style lj/class2/gpu command
================================
pair_style lj/class2/kk command
===============================
pair_style lj/class2/omp command
================================
pair_style lj/class2/coul/cut command
=====================================
pair_style lj/class2/coul/cut/kk command
========================================
pair_style lj/class2/coul/cut/omp command
=========================================
pair_style lj/class2/coul/long command
======================================
pair_style lj/class2/coul/long/gpu command
==========================================
pair_style lj/class2/coul/long/kk command
=========================================
pair_style lj/class2/coul/long/omp command
==========================================
Syntax
""""""
.. parsed-literal::
pair_style style args
* style = *lj/class2* or *lj/class2/coul/cut* or *lj/class2/coul/long*
* args = list of arguments for a particular style
.. parsed-literal::
*lj/class2* args = cutoff
cutoff = global cutoff for class 2 interactions (distance units)
*lj/class2/coul/cut* args = cutoff (cutoff2)
cutoff = global cutoff for class 2 (and Coulombic if only 1 arg) (distance units)
cutoff2 = global cutoff for Coulombic (optional) (distance units)
*lj/class2/coul/long* args = cutoff (cutoff2)
cutoff = global cutoff for class 2 (and Coulombic if only 1 arg) (distance units)
cutoff2 = global cutoff for Coulombic (optional) (distance units)
Examples
""""""""
.. parsed-literal::
pair_style lj/class2 10.0
pair_coeff * * 100.0 2.5
pair_coeff 1 2* 100.0 2.5 9.0
.. parsed-literal::
pair_style lj/class2/coul/cut 10.0
pair_style lj/class2/coul/cut 10.0 8.0
pair_coeff * * 100.0 3.0
pair_coeff 1 1 100.0 3.5 9.0
pair_coeff 1 1 100.0 3.5 9.0 9.0
.. parsed-literal::
pair_style lj/class2/coul/long 10.0
pair_style lj/class2/coul/long 10.0 8.0
pair_coeff * * 100.0 3.0
pair_coeff 1 1 100.0 3.5 9.0
Description
"""""""""""
The *lj/class2* styles compute a 6/9 Lennard-Jones potential given by
.. image:: Eqs/pair_class2.jpg
:align: center
Rc is the cutoff.
The *lj/class2/coul/cut* and *lj/class2/coul/long* styles add a
Coulombic term as described for the :doc:`lj/cut <pair_lj>` pair styles.
See :ref:`(Sun) <pair-Sun>` for a description of the COMPASS class2 force field.
The following coefficients must be defined for each pair of atoms
types via the :doc:`pair_coeff <pair_coeff>` command as in the examples
above, or in the data file or restart files read by the
:doc:`read_data <read_data>` or :doc:`read_restart <read_restart>`
commands, or by mixing as described below:
* epsilon (energy units)
* sigma (distance units)
* cutoff1 (distance units)
* cutoff2 (distance units)
The latter 2 coefficients are optional. If not specified, the global
class 2 and Coulombic cutoffs are used. If only one cutoff is
specified, it is used as the cutoff for both class 2 and Coulombic
interactions for this type pair. If both coefficients are specified,
they are used as the class 2 and Coulombic cutoffs for this type pair.
You cannot specify 2 cutoffs for style *lj/class2*\ , since it has no
Coulombic terms.
For *lj/class2/coul/long* only the class 2 cutoff can be specified
since a Coulombic cutoff cannot be specified for an individual I,J
type pair. All type pairs use the same global Coulombic cutoff
specified in the pair_style command.
----------
If the pair_coeff command is not used to define coefficients for a
particular I != J type pair, the mixing rule for epsilon and sigma for
all class2 potentials is to use the *sixthpower* formulas documented
by the :doc:`pair_modify <pair_modify>` command. The :doc:`pair_modify mix <pair_modify>` setting is thus ignored for class2 potentials
for epsilon and sigma. However it is still followed for mixing the
cutoff distance.
----------
Styles with a *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 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.
----------
**Mixing, shift, table, tail correction, restart, rRESPA info**\ :
For atom type pairs I,J and I != J, the epsilon and sigma coefficients
and cutoff distance for all of the lj/class2 pair styles can be mixed.
Epsilon and sigma are always mixed with the value *sixthpower*\ . The
cutoff distance is mixed by whatever option is set by the pair_modify
command (default = geometric). See the "pair_modify" command for
details.
All of the lj/class2 pair styles support the
:doc:`pair_modify <pair_modify>` shift option for the energy of the
Lennard-Jones portion of the pair interaction.
The *lj/class2/coul/long* pair style does not support the
:doc:`pair_modify <pair_modify>` table option since a tabulation
capability has not yet been added to this potential.
All of the lj/class2 pair styles support the
:doc:`pair_modify <pair_modify>` tail option for adding a long-range
tail correction to the energy and pressure of the Lennard-Jones
portion of the pair interaction.
All of the lj/class2 pair styles write their information to :doc:`binary restart files <restart>`, so pair_style and pair_coeff commands do
not need to be specified in an input script that reads a restart file.
All of the lj/class2 pair styles can only be used via the *pair*
keyword of the :doc:`run_style respa <run_style>` command. They do not
support the *inner*\ , *middle*\ , *outer* keywords.
Restrictions
""""""""""""
These styles are part of the CLASS2 package. They are only enabled if
LAMMPS was built with that package. See the :ref:`Making LAMMPS <start_3>` section for more info.
Related commands
""""""""""""""""
:doc:`pair_coeff <pair_coeff>`
**Default:** none
----------
.. _pair-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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