.. index:: pair_style soft
.. index:: pair_style soft/gpu
.. index:: pair_style soft/kk
.. index:: pair_style soft/omp

pair_style soft command
=======================

Accelerator Variants: *soft/gpu*, *soft/kk*, *soft/omp*

Syntax
""""""

.. code-block:: LAMMPS

   pair_style soft cutoff

* cutoff = global cutoff for soft interactions (distance units)

Examples
""""""""

.. code-block:: LAMMPS

   pair_style soft 1.0
   pair_coeff * * 10.0
   pair_coeff 1 1 10.0 3.0

   pair_style soft 1.0
   pair_coeff * * 0.0
   variable prefactor equal ramp(0,30)
   fix 1 all adapt 1 pair soft a * * v_prefactor

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

Style *soft* computes pairwise interactions with the formula

.. math::

   E = A \left[ 1 + \cos\left(\frac{\pi r}{r_c}\right) \right]
   \qquad r < r_c

It is useful for pushing apart overlapping atoms, since it does not
blow up as r goes to 0.  A is a prefactor that can be made to vary in
time from the start to the end of the run (see discussion below),
e.g. to start with a very soft potential and slowly harden the
interactions over time.  :math:`r_c` is the cutoff.
See the :doc:`fix nve/limit <fix_nve_limit>` command for another way
to push apart overlapping atoms.

The following coefficients must be defined for each pair of atom 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:

* A (energy units)
* cutoff (distance units)

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

.. note::

   The syntax for :doc:`pair_coeff <pair_coeff>` with a single A
   coeff is different in the current version of LAMMPS than in older
   versions which took two values, Astart and Astop, to ramp between
   them.  This functionality is now available in a more general form
   through the :doc:`fix adapt <fix_adapt>` command, as explained below.
   Note that if you use an old input script and specify Astart and Astop
   without a cutoff, then LAMMPS will interpret that as A and a cutoff,
   which is probably not what you want.

The :doc:`fix adapt <fix_adapt>` command can be used to vary A for one
or more pair types over the course of a simulation, in which case
pair_coeff settings for A must still be specified, but will be
overridden.  For example these commands will vary the prefactor A for
all pairwise interactions from 0.0 at the beginning to 30.0 at the end
of a run:

.. code-block:: LAMMPS

   variable prefactor equal ramp(0,30)
   fix 1 all adapt 1 pair soft a * * v_prefactor

Note that a formula defined by an :doc:`equal-style variable <variable>`
can use the current timestep, elapsed time in the current run, elapsed
time since the beginning of a series of runs, as well as access other
variables.

----------

.. include:: accel_styles.rst

----------

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

For atom type pairs I,J and I != J, the A coefficient and cutoff
distance for this pair style can be mixed.  A is always mixed via a
*geometric* rule.  The cutoff is mixed according to the pair_modify
mix value.  The default mix value is *geometric*\ .  See the
"pair_modify" command for details.

This pair style does not support the :doc:`pair_modify <pair_modify>`
shift option, since the pair interaction goes to 0.0 at the cutoff.

The :doc:`pair_modify <pair_modify>` table and tail options are not
relevant for this pair style.

This pair style writes its 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.

This pair style can only be used via the *pair* keyword of the
:doc:`run_style respa <run_style>` command.  It does not support the
*inner*, *middle*, *outer* keywords.

----------

Restrictions
""""""""""""
 none

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

:doc:`pair_coeff <pair_coeff>`, :doc:`fix nve/limit <fix_nve_limit>`, :doc:`fix adapt <fix_adapt>`

Default
"""""""

none
