pair_style soft command¶
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.
Warning
The syntax for pair_coeff with a single A coeff is different in the current version of LIGGGHTS(R)-PUBLIC 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 LIGGGHTS(R)-PUBLIC 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