Syntax:
pair_style sw
Examples:
pair_style sw pair_coeff * * si.sw Si pair_coeff * * SiC.sw Si C Si
Description:
The sw style computes a 3-body Stillinger-Weber potential for the energy E of a system of atoms as
where phi2 is a two-body term and phi3 is a three-body term. The summations in the formula are over all neighbors J and K of atom I within a cutoff distance = a*sigma.
Only a single pair_coeff command is used with the sw style which specifies a Stillinger-Weber potential file with parameters for all needed elements. These are mapped to LAMMPS atom types by specifying N additional arguments after the filename in the pair_coeff command, where N is the number of LAMMPS atom types:
As an example, imagine the SiC.sw file has Stillinger-Weber values for Si and C. If your LAMMPS simulation has 4 atoms types and you want the 1st 3 to be Si, and the 4th to be C, you would use the following pair_coeff command:
pair_coeff * * SiC.sw Si Si Si C
The 1st 2 arguments must be * * so as to span all LAMMPS atom types. The first three Si arguments map LAMMPS atom types 1,2,3 to the Si element in the SW file. The final C argument maps LAMMPS atom type 4 to the C element in the SW file. If a mapping value is specified as NULL, the mapping is not performed. This can be used when a sw potential is used as part of the hybrid pair style. The NULL values are placeholders for atom types that will be used with other potentials.
Stillinger-Weber files in the potentials directory of the LAMMPS distribution have a ".sw" suffix. Lines that are not blank or comments (starting with #) define parameters for a triplet of elements. The parameters in a single entry correspond to the two-body and three-body coefficients in the formula above:
The epsilon, sigma, a, A, B, p, and q parameters are for two-body interactions. The lambda, gamma, and costheta0 parameters are for three-body interactions. The non-annotated parameters are unitless.
The Stillinger-Weber potential file must contain entries for all the elements listed in the pair_coeff command. It can also contain entries for additional elements not being used in a particular simulation; LAMMPS ignores those entries.
For a single-element simulation, only a single entry is required (e.g. SiSiSi). For a two-element simulation, the file must contain 8 entries (for SiSiSi, SiSiC, SiCSi, SiCC, CSiSi, CSiC, CCSi, CCC), that specify SW parameters for all permutations of the two elements interacting in three-body configurations. Thus for 3 elements, 27 entries would be required, etc.
As annotated above, the first element in the entry is the center atom in a three-body interaction. Thus an entry for SiCC means a Si atom with 2 C atoms as neighbors. By symmetry, three-body parameters for SiCSi and SiSiC entries should be the same. Two-body parameters for an interaction come from the entry where the 2nd element is repeated. Thus the two-body parameters for Si interacting with C, comes from the SiCC entry. Again by symmetry, the two-body parameters in the SiCC and CSiSi entries should thus be the same. Two-body parameters in entries whose 2nd and 3rd element are different (e.g. SiCSi) are ignored.
Restrictions:
This pair potential requires the newton setting to be "on" for pair interactions.
Related commands:
Default: none
(Stillinger) Stillinger and Weber, Phys Rev B, 31, 5262 (1985).