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.. index:: pair_style nb3b/harmonic
pair_style nb3b/harmonic command
================================
pair_style nb3b/harmonic/omp command
====================================
Syntax
""""""
.. parsed-literal::
pair_style nb3b/harmonic
Examples
""""""""
.. parsed-literal::
pair_style nb3b/harmonic
pair_coeff * * MgOH.nb3bharmonic Mg O H
Description
"""""""""""
This pair style computes a nonbonded 3-body harmonic potential for the
energy E of a system of atoms as
.. image:: Eqs/pair_nb3b_harmonic.jpg
:align: center
where *theta_0* is the equilibrium value of the angle and *K* is a
prefactor. Note that the usual 1/2 factor is included in *K*. The form
of the potential is identical to that used in angle_style *harmonic*,
but in this case, the atoms do not need to be explicitly bonded.
Only a single pair_coeff command is used with this style which
specifies a potential file with parameters for specified elements.
These are mapped to LAMMPS atom types by specifying N additional
arguments after the filename in the pair_coeff command, where N is the
number of LAMMPS atom types:
* filename
* N element names = mapping of elements to atom types
See the :doc:`pair_coeff <pair_coeff>` doc page for alternate ways
to specify the path for the potential file.
As an example, imagine a file SiC.nb3b.harmonic has potential values
for Si and C. If your LAMMPS simulation has 4 atoms types and you
want the 1st 3 to be Si, and the 4th to be C, you would use the
following pair_coeff command:
.. parsed-literal::
pair_coeff * * SiC.nb3b.harmonic Si Si Si C
The 1st 2 arguments must be * * so as to span all LAMMPS atom types.
The first three Si arguments map LAMMPS atom types 1,2,3 to the Si
element in the potential file. The final C argument maps LAMMPS atom
type 4 to the C element in the potential file. If a mapping value is
specified as NULL, the mapping is not performed. This can be used
when the potential is used as part of the *hybrid* pair style. The
NULL values are placeholders for atom types that will be used with
other potentials. An example of a pair_coeff command for use with the
*hybrid* pair style is:
pair_coeff * * nb3b/harmonic MgOH.nb3b.harmonic Mg O H
Three-body nonbonded harmonic files in the *potentials* directory of
the LAMMPS distribution have a ".nb3b.harmonic" suffix. Lines that
are not blank or comments (starting with #) define parameters for a
triplet of elements.
Each entry has six arguments. The first three are atom types as
referenced in the LAMMPS input file. The first argument specifies the
central atom. The fourth argument indicates the *K* parameter. The
fifth argument indicates *theta_0*. The sixth argument indicates a
separation cutoff in Angstroms.
For a given entry, if the second and third arguments are identical,
then the entry is for a cutoff for the distance between types 1 and 2
(values for *K* and *theta_0* are irrelevant in this case).
For a given entry, if the first three arguments are all different,
then the entry is for the *K* and *theta_0* parameters (the cutoff in
this case is irrelevant).
It is *not* required that the potential file contain entries for all
of the elements listed in the pair_coeff command. It can also contain
entries for additional elements not being used in a particular
simulation; LAMMPS ignores those entries.
----------
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 pair style can only be used if LAMMPS was built with the MANYBODY
package (which it is by default). See the :ref:`Making LAMMPS <start_3>` section for more info on packages.
Related commands
""""""""""""""""
:doc:`pair_coeff <pair_coeff>`
**Default:** none
.. _lws: http://lammps.sandia.gov
.. _ld: Manual.html
.. _lc: Section_commands.html#comm
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