# pair_coeff command

## Syntax

    pair_coeff I J args

- I,J = atom types (see asterisk form below)
- args = coefficients for one or more pairs of atom types

## 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. LIGGGHTS(R)-PUBLIC 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.

Many pair styles, typically for many-body potentials, use tabulated potential files as input, when specifying the pair_coeff command. Potential files provided with LIGGGHTS(R)-PUBLIC are in the potentials directory of the distribution. For some potentials, such as EAM, other archives of suitable files can be found on the Web. They can be used with LIGGGHTS(R)-PUBLIC so long as they are in the format LIGGGHTS(R)-PUBLIC expects, as discussed on the individual doc pages.

When a pair_coeff command using a potential file is specified, LIGGGHTS(R)-PUBLIC looks for the potential file in 2 places. First it looks in the location specified. E.g. if the file is specified as “niu3.eam”, it is looked for in the current working directory. If it is specified as ”../potentials/niu3.eam”, then it is looked for in the potentials directory, assuming it is a sister directory of the current working directory. If the file is not found, it is then looked for in the directory specified by the LAMMPS_POTENTIALS environment variable. Thus if this is set to the potentials directory in the LIGGGHTS(R)-PUBLIC distro, then you can use those files from anywhere on your system, without copying them into your working directory. Environment variables are set in different ways for different shells. Here are example settings for

    csh, tcsh:
    % setenv LAMMPS_POTENTIALS /path/to/lammps/potentials

    bash:
    % export LAMMPS_POTENTIALS=/path/to/lammps/potentials

    Windows:
    % set LAMMPS_POTENTIALS="C:Path to LAMMPSPotentials

The full list of pair styles defined in LIGGGHTS(R)-PUBLIC is on 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