LAMMPS WWW Site - LAMMPS Documentation - LAMMPS Commands

### 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. LAMMPS 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

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:

- pair_style hybrid - multiple styles of pairwise interactions
- pair_style hybrid/overlay - multiple styles of superposed pairwise interactions

- pair_style airebo - AI-REBO potential
- pair_style born/coul/long - Born-Mayer-Huggins with long-range Coulomb
- pair_style buck - Buckingham potential
- pair_style buck/coul/cut - Buckingham with cutoff Coulomb
- pair_style buck/coul/long - Buckingham with long-range Coulomb
- pair_style colloid - integrated colloidal potential
- pair_style coul/cut - cutoff Coulombic potential
- pair_style coul/debye - cutoff Coulombic potential with Debye screening
- pair_style coul/long - long-range Coulombic potential
- pair_style dipole/cut - point dipoles with cutoff
- pair_style dpd - dissipative particle dynamics (DPD)
- pair_style dsmc - Direct Simulation Monte Carlo (DSMC)
- pair_style eam - embedded atom method (EAM)
- pair_style eam/opt - optimized version of EAM
- pair_style eam/alloy - alloy EAM
- pair_style eam/alloy/opt - optimized version of alloy EAM
- pair_style eam/fs - Finnis-Sinclair EAM
- pair_style eam/fs/opt - optimized version of Finnis-Sinclair EAM
- pair_style gayberne - Gay-Berne ellipsoidal potential
- pair_style gayberne/gpu - GPU-enabled Gay-Berne ellipsoidal potential
- pair_style gran/hertz/history - granular potential with Hertzian interactions
- pair_style gran/hooke - granular potential with history effects
- pair_style gran/hooke/history - granular potential without history effects
- pair_style lj/charmm/coul/charmm - CHARMM potential with cutoff Coulomb
- pair_style lj/charmm/coul/charmm/implicit - CHARMM for implicit solvent
- pair_style lj/charmm/coul/long - CHARMM with long-range Coulomb
- pair_style lj/charmm/coul/long/opt - optimized version of CHARMM with long-range Coulomb
- pair_style lj/class2 - COMPASS (class 2) force field with no Coulomb
- pair_style lj/class2/coul/cut - COMPASS with cutoff Coulomb
- pair_style lj/class2/coul/long - COMPASS with long-range Coulomb
- pair_style lj/cut - cutoff Lennard-Jones potential with no Coulomb
- pair_style lj/cut/gpu - GPU-enabled version of cutoff LJ
- pair_style lj/cut/opt - optimized version of cutoff LJ
- pair_style lj/cut/coul/cut - LJ with cutoff Coulomb
- pair_style lj/cut/coul/debye - LJ with Debye screening added to Coulomb
- pair_style lj/cut/coul/long - LJ with long-range Coulomb
- pair_style lj/cut/coul/long/tip4p - LJ with long-range Coulomb for TIP4P water
- pair_style lj/expand - Lennard-Jones for variable size particles
- pair_style lj/gromacs - GROMACS-style Lennard-Jones potential
- pair_style lj/gromacs/coul/gromacs - GROMACS-style LJ and Coulombic potential
- pair_style lj/smooth - smoothed Lennard-Jones potential
- pair_style lj96/cut - Lennard-Jones 9/6 potential
- pair_style lubricate - hydrodynamic lubrication forces
- pair_style meam - modified embedded atom method (MEAM)
- pair_style morse - Morse potential
- pair_style morse/opt - optimized version of Morse potential
- pair_style peri/pmb - peridynamic PMB potential
- pair_style reax - ReaxFF potential
- pair_style resquared - Everaers RE-Squared ellipsoidal potential
- pair_style soft - Soft (cosine) potential
- pair_style sw - Stillinger-Weber 3-body potential
- pair_style table - tabulated pair potential
- pair_style tersoff - Tersoff 3-body potential
- pair_style tersoff/zbl - Tersoff/ZBL 3-body potential
- pair_style yukawa - Yukawa potential
- pair_style yukawa/colloid - screened Yukawa potential for finite-size particles

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.

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