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### pair_style soft command

Syntax:

    pair_style soft cutoff

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

Examples:

    pair_style soft 2.5
    pair_coeff * * 10.0
    pair_coeff 1 1 10.0 3.0

    pair_style soft 2.5
    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

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 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 pair_coeff command as in the examples above, or in the data file or restart files read by the read_data or 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.

IMPORTANT NOTE: The syntax for 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 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 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:

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

Note that a formula defined by an equal-style 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.

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 pair_modify shift option, since the pair interaction goes to 0.0 at the cutoff.

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

This pair style writes its information to binary restart files, 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 command. It does not support the inner, middle, outer keywords.

Restrictions: none

Related commands:

pair_coeff, fix nve/limit, fix adapt

Default: none