lammps/doc/pair_soft.txt

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pair_style soft command :h3

[Syntax:]

pair_style soft cutoff :pre

cutoff = global cutoff for soft interactions (distance units) :ul

[Examples:]

pair_style soft 2.5
pair_coeff * * 0.0 60.0
pair_coeff 1 1 0.0 60.0 3.0 :pre

[Description:]

Style {soft} computes pairwise interactions with the formula

:c,image(Eqs/pair_soft.jpg)

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 varies in time from
the start to the end of the run.  The "run"_run.html command documents
how to make the ramping take place across multiple runs.  Rc is the
cutoff.  See the "fix nve/limit"_fix_nve_limit.html command for
another way to push apart overlapping atoms.

The following coefficients must be defined for each pair of atoms
types via the "pair_coeff"_pair_coeff.html command as in the examples
above, or in the data file or restart files read by the
"read_data"_read_data.html or "read_restart"_read_restart.html
commands, or by mixing as described below:

Astart (energy units)
Astop (energy units)
cutoff (distance units) :ul

Astart and Astop are the values of the prefactor at the start and end
of the next run.  At intermediate times the value of A will be ramped
between these 2 values.  Note that before performing a 2nd run, you
will want to adjust the values of Astart and Astop for all type pairs,
or switch to a new pair style.

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

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[Mixing, shift, table, tail correction, per-atom energy/stress,
restart, rRESPA info]:

For atom type pairs I,J and I != J, the Astart, Astop coefficients and
cutoff distance for this pair style can be mixed.  Astart and Atop are
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 styles does not support the "pair_modify"_pair_modify.html
shift option, since the pair interaction is goes to 0.0 at the cutoff.

The "pair_modify"_pair_modify.html table and tail options are not
relevant for this pair style.

This pair style can calculate per-atom energy and stress, as used by
the "compute epair/atom"_compute_epair_atom.html, "compute
stress/atom"_compute_stress_atom.html, and "dump custom"_dump.html
commands.

This pair style writes its information to "binary restart
files"_restart.html, 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
"run_style respa"_run_style.html command.  It does not support the
{inner}, {middle}, {outer} keywords.

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[Restrictions:] none

[Related commands:]

"pair_coeff"_pair_coeff.html, "fix nve/limit"_fix_nve_limit.html

[Default:] none