.. index:: pair_style morse

pair_style morse command
========================

pair_style morse/gpu command
============================

pair_style morse/omp command
============================

pair_style morse/opt command
============================

pair_style morse/smooth/linear command
======================================

pair_style morse/smooth/linear/omp command
==========================================

pair_style morse/soft command
=============================

Syntax
""""""

.. parsed-literal::

   pair_style style args

* style = *morse* or *morse/smooth/linear* or *morse/soft*
* args = list of arguments for a particular style

.. parsed-literal::

    *morse* args = cutoff
      cutoff = global cutoff for Morse interactions (distance units)
    *morse/smooth/linear* args = cutoff
      cutoff = global cutoff for Morse interactions (distance units)
    *morse/soft* args = n lf cutoff
      n = soft-core parameter
      lf      = transformation range is lf < lambda < 1
      cutoff  = global cutoff for Morse interactions (distance units)

Examples
""""""""

pair_style morse 2.5
pair_style morse/smooth/linear 2.5
pair_coeff * * 100.0 2.0 1.5
pair_coeff 1 1 100.0 2.0 1.5 3.0

.. parsed-literal::

   pair_style morse/soft 4 0.9 10.0
   pair_coeff * * 100.0 2.0 1.5 1.0
   pair_coeff 1 1 100.0 2.0 1.5 1.0 3.0

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

Style *morse* computes pairwise interactions with the formula

.. image:: Eqs/pair_morse.jpg
   :align: center

Rc is the cutoff.

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:

* D0 (energy units)
* alpha (1/distance units)
* r0 (distance units)
* cutoff (distance units)

The last coefficient is optional.  If not specified, the global morse
cutoff is used.


----------


The *morse/smooth/linear* variant is similar to the lj/smooth/linear
variant in that it adds to the potential a shift and a linear term
so that both, potential energy and force, go to zero at the cut-off:

.. image:: Eqs/pair_morse_smooth_linear.jpg
   :align: center

The syntax of the pair_style and pair_coeff commands are the same for
the *morse* and *morse/smooth/linear* styles.


----------


The *morse/soft* variant is similar to the *lj/cut/soft* pair style
in that it modifies the potential at short range to have a soft core.
This helps to avoid singularities during free energy calculation in
which sites are created or anihilated. The formula differs from that
of *lj/cut/soft*\ , and is instead given by:

.. image:: Eqs/pair_morse_soft.jpg
   :align: center

The *morse/soft* style requires the following pair coefficients:

* D0 (energy units)
* alpha (1/distance units)
* r0 (distance units)
* lamda (unitless, between 0.0 and 1.0)
* cutoff (distance units)

The last coefficient is optional.  If not specified, the global morse
cutoff is used.


----------


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**\ :

None of these pair styles support mixing.  Thus, coefficients for all
I,J pairs must be specified explicitly.

All of these pair styles support the :doc:`pair_modify <pair_modify>`
shift option for the energy of the pair interaction.

The :doc:`pair_modify <pair_modify>` table options is not relevant for
the Morse pair styles.

None of these pair styles support the :doc:`pair_modify <pair_modify>`
tail option for adding long-range tail corrections to energy and
pressure.

All of these 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.

These 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
""""""""""""


The *morse/smooth/linear* pair style is only enabled if LAMMPS was
built with the USER-MISC package.  See the :ref:`Making LAMMPS <start_3>` section for more info.

The *morse/soft* pair style is only enabled if LAMMPS was built with
the USER-FEP package.  See the :ref:`Making LAMMPS <start_3>` section for more info.

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

:doc:`pair_coeff <pair_coeff>`

**Default:** none


.. _lws: http://lammps.sandia.gov
.. _ld: Manual.html
.. _lc: Section_commands.html#comm