.. index:: pair_style soft pair_style soft command ======================= Syntax """""" .. parsed-literal:: pair_style soft cutoff * cutoff = global cutoff for soft interactions (distance units) Examples """""""" .. parsed-literal:: pair_style soft 2.5 pair_coeff * * 10.0 pair_coeff 1 1 10.0 3.0 .. parsed-literal:: 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 .. 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 ` 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 ` command as in the examples above, or in the data file or restart files read by the :doc:`read_data ` or :doc:`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 :doc:`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 :doc:`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 :doc:`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 ` 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 :doc:`pair_modify ` shift option, since the pair interaction goes to 0.0 at the cutoff. The :doc:`pair_modify ` table and tail options are not relevant for this pair style. This pair style writes its information to :doc:`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 :doc:`run_style respa ` command. It does not support the *inner*, *middle*, *outer* keywords. ---------- Restrictions """""""""""" none Related commands """""""""""""""" :doc:`pair_coeff `, :doc:`fix nve/limit `, :doc:`fix adapt ` **Default:** none .. _liws: http://www.cfdem.com .. _ld: Manual.html .. _lc: Section_commands.html#comm