Syntax:
pair_coeff I J args
Examples:
pair_coeff 1 2 1.0 1.0 2.5 pair_coeff 2 * 1.0 1.0 pair_coeff 3* 1*2 1.0 1.0 2.5 pair_coeff * * 1.0 1.0 pair_coeff * * nialhjea 1 1 2 pair_coeff * 3 morse.table ENTRY1 pair_coeff 1 2 lj/cut 1.0 1.0 2.5 (for pair_style hybrid)
Description:
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. Pair coefficients can also be set in the data file read by the read_data command or in a restart file.
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. LAMMPS sets the coefficients for the symmetric J,I interaction to the same values.
A wildcard 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
A line in a data file that specifies pair coefficients uses the exact same format as the arguments of the pair_coeff command in an input script, with the exception of the I,J type arguments. In each line of the "Pair Coeffs" section of a data file, only a single type I is specified, which sets the coefficients for type I interacting with type I. This is because the section has exactly N lines, where N = the number of atom types. For this reason, the wild-card asterisk should also not be used as part of the I argument. Thus in a data file, the line corresponding to the 1st example above would be listed as
2 1.0 1.0 2.5
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; see the pair_modify command for a discussion. Details on this option as it pertains to individual potentials are described on the doc page for the potential.
Here is an alphabetic list of pair styles defined in LAMMPS. 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.
Note that there are also additional pair styles submitted by users which are included in the LAMMPS distribution. The list of these with links to the individual styles are given in the pair section of this page.
There are also additional accelerated pair styles included in the LAMMPS distribution for faster performance on CPUs and GPUs. The list of these with links to the individual styles are given in the pair section of this page.
Restrictions:
This command must come after the simulation box is defined by a read_data, read_restart, or create_box command.
Related commands:
pair_style, pair_modify, read_data, read_restart, pair_write
Default: none