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.. index:: pair_style soft
pair_style soft command
=======================
pair_style soft/gpu command
===========================
pair_style soft/omp command
===========================
Syntax
""""""
.. parsed-literal::
pair_style soft cutoff
* cutoff = global cutoff for soft interactions (distance units)
Examples
""""""""
.. parsed-literal::
pair_style soft 1.0
pair_coeff * * 10.0
pair_coeff 1 1 10.0 3.0
.. parsed-literal::
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
.. image:: Eqs/pair_soft.jpg
:align: center
It is useful for pushing apart overlapping atoms, since it does not
blow up as r goes to 0. A is a pre-factor 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. Rc 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 probabably 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:
.. parsed-literal::
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.
----------
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 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
.. _lws: http://lammps.sandia.gov
.. _ld: Manual.html
.. _lc: Section_commands.html#comm
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