\(\renewcommand{\AA}{\text{Å}}\)

# pair_coeff command

## Syntax

    pair_coeff I J args

- I,J = numeric atom types (see asterisk form below), or type labels

- 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)

    labelmap atom 1 C
    labelmap atom 2 H
    pair_coeff C H 1.0 1.0 2.5

## 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 several ways. Explicit numeric values can be used for each, as in the first example above. Or, one or both of the types in the I,J pair can be a type label, which is an alphanumeric string defined by the labelmap command or in a section of a data file read by the read_data command, and which converts internally to a numeric type. Internally, LAMMPS will set coefficients for the symmetric J,I interaction to the same values as the I,J interaction.

For numeric values only, 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 \(N\) is 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). For the asterisk syntax, only type pairs with I <= J are considered; if asterisks imply type pairs where J < I, they are ignored. Again internally, LAMMPS will set the coefficients for the symmetric J,I interactions to the same values as the I <= J interactions.

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 \(N\) is 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 first 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 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 LAMMPS 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 LAMMPS so long as they are in the format LAMMPS expects, as discussed on the individual doc pages. The first line of potential files may contain metadata with upper case tags followed their value. These may be parsed and used by LAMMPS. Currently supported are the “DATE:” tag and the `UNITS:` tag. For pair styles that have been programmed to support the metadata, the value of the “DATE:” tag is printed to the screen and logfile so that the version of a potential file can be later identified. The `UNITS:` tag indicates the units setting required for this particular potential file. If the potential file was created for a different sets of units, LAMMPS will terminate with an error. If the potential file does not contain the tag, no check will be made and it is the responsibility of the user to determine that the unit style is correct.

In some select cases and for specific combinations of unit styles, LAMMPS is capable of automatically converting potential parameters from a file. In those cases, a warning message signaling that an automatic conversion has happened is printed to the screen.

When a pair_coeff command using a potential file is specified, LAMMPS 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 one of the directories specified by the `LAMMPS_POTENTIALS` environment variable. Thus if this is set to the potentials directory in the LAMMPS distribution, 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 LAMMPS\\Potentials"

The `LAMMPS_POTENTIALS` environment variable may contain paths to multiple folders, if they are separated by “;” on Windows and “:” on all other operating systems, just like the `PATH` and similar environment variables.

The alphabetic list of pair styles defined in LAMMPS is given on the pair_style doc page. They are also listed in more compact form on the Commands pair doc page.

Click on the style to display the formula it computes and its coefficients as specified by the associated pair_coeff command.

## 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