.. index:: pair_style nm/cut
.. index:: pair_style nm/cut/split
.. index:: pair_style nm/cut/coul/cut
.. index:: pair_style nm/cut/coul/long
.. index:: pair_style nm/cut/omp
.. index:: pair_style nm/cut/coul/cut/omp
.. index:: pair_style nm/cut/coul/long/omp

pair_style nm/cut command
=========================

Accelerator Variants: *nm/cut/omp*

pair_style nm/cut/split command
===============================

pair_style nm/cut/coul/cut command
==================================

Accelerator Variants: *nm/cut/coul/cut/omp*

pair_style nm/cut/coul/long command
===================================

Accelerator Variants: *nm/cut/coul/long/omp*

Syntax
""""""

.. code-block:: LAMMPS

   pair_style style args

* style = *nm/cut* or *nm/cut/split* or *nm/cut/coul/cut* or *nm/cut/coul/long*
* args = list of arguments for a particular style

  .. parsed-literal::

       *nm/cut* args = cutoff
         cutoff = global cutoff for Pair interactions (distance units)
       *nm/cut/split* args = cutoff
         cutoff = global cutoff for Pair interactions (distance units)
       *nm/cut/coul/cut* args = cutoff (cutoff2)
         cutoff = global cutoff for Pair (and Coulombic if only 1 arg) (distance units)
         cutoff2 = global cutoff for Coulombic (optional) (distance units)
       *nm/cut/coul/long* args = cutoff (cutoff2)
         cutoff = global cutoff for Pair (and Coulombic if only 1 arg) (distance units)
         cutoff2 = global cutoff for Coulombic (optional) (distance units)

Examples
""""""""

.. code-block:: LAMMPS

   pair_style nm/cut 12.0
   pair_coeff * * 0.01 5.4 8.0 7.0
   pair_coeff 1 1 0.01 4.4 7.0 6.0

   pair_style nm/cut/split 1.12246
   pair_coeff 1 1 1.0 1.1246 12 6
   pair_coeff * * 1.0 1.1246 11 6

   pair_style nm/cut/coul/cut 12.0 15.0
   pair_coeff * * 0.01 5.4 8.0 7.0
   pair_coeff 1 1 0.01 4.4 7.0 6.0

   pair_style nm/cut/coul/long 12.0 15.0
   pair_coeff * * 0.01 5.4 8.0 7.0
   pair_coeff 1 1 0.01 4.4 7.0 6.0

Description
"""""""""""

Style *nm* computes site-site interactions based on the N-M potential
by :ref:`Clarke <Clarke>`, mainly used for ionic liquids.  A site can
represent a single atom or a united-atom site.  The energy of an
interaction has the following form:

.. math::

   E = \frac{E_0}{(n-m)} \left[ m \left(\frac{r_0}{r}\right)^n - n
   \left(\frac{r_0}{r}\right)^m \right] \qquad r < r_c

where :math:`r_c` is the cutoff and :math:`r_0` is the minimum of the
potential.  Please note that this differs from the convention used for
other Lennard-Jones potentials in LAMMPS where :math:`\sigma` represents
the location where the energy is zero.

Style *nm/cut/split* applies the standard LJ (12-6) potential above
:math:`r_0 = 2^\frac{1}{6}\sigma`.  Style *nm/cut/split* is employed in
polymer equilibration protocols that combine core-softening approaches
with topology-changing moves :ref:`Dietz <Dietz>`.

Style *nm/cut/coul/cut* adds a Coulombic pairwise interaction given by

.. math::

   E = \frac{C q_i q_j}{\epsilon  r} \qquad r < r_c

where :math:`C` is an energy-conversion constant, :math:`q_i` and :math:`q_j`
are the charges on the two atoms, and epsilon is the dielectric constant which can
be set by the :doc:`dielectric <dielectric>` command.  If one cutoff is
specified in the pair_style command, it is used for both the N-M and Coulombic
terms.  If two cutoffs are specified, they are used as cutoffs for the N-M and
Coulombic terms respectively.

Styles *nm/cut/coul/long* compute the same
Coulombic interactions as style *nm/cut/coul/cut* except that an
additional damping factor is applied to the Coulombic term so it can
be used in conjunction with the :doc:`kspace_style <kspace_style>`
command and its *ewald* or *pppm* option.  The Coulombic cutoff
specified for this style means that pairwise interactions within this
distance are computed directly; interactions outside that distance are
computed in reciprocal space.

For all of the *nm* pair styles, 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.

* :math:`E_0` (energy units)
* :math:`r_0` (distance units)
* :math:`n` (unitless)
* :math:`m` (unitless)
* cutoff1 (distance units)
* cutoff2 (distance units)

The latter 2 coefficients are optional.  If not specified, the global
N-M and Coulombic cutoffs specified in the pair_style command are used.
If only one cutoff is specified, it is used as the cutoff for both N-M
and Coulombic interactions for this type pair.  If both coefficients
are specified, they are used as the N-M and Coulombic cutoffs for this
type pair.  You cannot specify 2 cutoffs for style *nm*, since it
has no Coulombic terms.

For *nm/cut/coul/long* only the N-M 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.

----------

Mixing, shift, table, tail correction, restart, rRESPA info
"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""

These pair styles do not support mixing. Thus, coefficients for all
I,J pairs must be specified explicitly.

All of the *nm* pair styles supports the
:doc:`pair_modify <pair_modify>` shift option for the energy of the pair
interaction.

The *nm/cut/coul/long* pair styles support the
:doc:`pair_modify <pair_modify>` table option since they can tabulate
the short-range portion of the long-range Coulombic interaction.

All of the *nm* pair styles support the :doc:`pair_modify <pair_modify>`
tail option for adding a long-range tail correction to the energy and
pressure for the N-M portion of the pair interaction.

All of the *nm* 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 *nm* 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.

----------

.. include:: accel_styles.rst

Restrictions
""""""""""""
These pair styles are part of the EXTRA-PAIR package.  They are only enabled if
LAMMPS was built with that package.  See the
:doc:`Build package <Build_package>` page for more info.

Related commands
""""""""""""""""

:doc:`pair_coeff <pair_coeff>`, :doc:`pair style lj/cut <pair_lj>`, :doc:`bond style fene/nm <bond_fene>`

Default
"""""""

none

----------

.. _Clarke:

**(Clarke)** Clarke and Smith, J Chem Phys, 84, 2290 (1986).

.. _Dietz:

**(Dietz)** Dietz and Hoy, J. Chem Phys, 156, 014103 (2022).
