pair_coeff I J args ...
pair_coeff 1 2 1.0 1.0 2.5 pair_coeff 2 * 1.0 1.0
Specify the pairwise force field coefficients for one or more pairs of atom types. The number and meaning of the coefficients depends on the pair style.
I and J can be specified in one of two ways. Explicit numeric values can be used for each, as in the 1st example above. I <= J is required. SPPARKS sets the coefficients for the symmetric J,I interaction to the same values.
A wild-card asterisk can be used in place of or in conjunction with the I,J arguments to set the coefficients for multiple pairs of atom types. This takes the form "*" or "*n" or "n*" or "m*n". If N = the number of atom types, then an asterisk with no numeric values means all types from 1 to N. A leading asterisk means all types from 1 to n (inclusive). A trailing asterisk means all types from n to N (inclusive). A middle asterisk means all types from m to n (inclusive). Note that only type pairs with I <= J are considered; if asterisks imply type pairs where J < I, they are ignored.
Note that a pair_coeff command can override a previous setting for the same I,J pair. For example, these commands set the coeffs for all I,J pairs, then overwrite the coeffs for just the I,J = 2,3 pair:
pair_coeff * * 1.0 1.0 2.5 pair_coeff 2 3 2.0 1.0 1.12
For many potentials, if coefficients for type pairs with I != J are not set explicitly by a pair_coeff command, the values are inferred from the I,I and J,J settings by mixing rules. Details on the mixing as it pertains to individual potentials are described on the doc page for the potential.
Here is the list of pair styles defined in SPPARKS. More will be added as new applications are developed. Click on the style to display the formula it computes, arguments specified in the pair_style command, and coefficients specified by the associated pair_coeff command: