lammps/doc/pair_coeff.txt

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pair_coeff command :h3

[Syntax:]

pair_coeff I J args :pre

I,J = atom types (see asterik form below)
args = coefficients for one or more pairs of atom types :ul

[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) :pre

[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"_read_data.html 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 asterik can be used in place of 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 asterik with no numeric values means all
types from 1 to N.  A leading asterik means all types from 1 to n
(inclusive).  A trailing asterik means all types from n to N
(inclusive).  A middle asterik means all types from m to n
(inclusive).  Note that only type pairs with I <= J are considered; if
asteriks 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 :pre

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 asterik
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 :pre

If coefficients for type pairs with I not equal J are not set
explicity by a pair_coeff command, they are inferred from the I,I and
J,J settings by mixing rules; see the "pair_modify"_pair_modify.html
command for a discussion.  Exceptions to the mixing rules of the
pair_modify command are discussed with the individual pair styles.

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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"_pair_coeff.html command:

"pair_style none"_pair_style_none.html - turn off pairwise interactions
"pair_style hybrid"_pair_style_hybrid.html - define multiple styles of pairwise interactions :ul

"pair_style buck"_pair_style_buck.html - Buckingham potential
"pair_style buck/coul/cut"_pair_style_buck.html - Buckinhham with cutoff Coulomb
"pair_style buck/coul/long"_pair_style_buck.html - Buckingham with long-range Coulomb
"pair_style dipole/cut"_pair_style_dipole.html - cutoff dipole and charge interactions
"pair_style dpd"_pair_style_dpd.html - dissipative particle dynamics (DPD)
"pair_style eam"_pair_style_eam.html - embedded atom method (EAM)
"pair_style eam/alloy"_pair_style_eam.html - alloy EAM
"pair_style eam/fs"_pair_style_eam.html - Finnis-Sinclair EAM
"pair_style gran/hertzian"_pair_style_granular.html - granular potential with Hertizain interactions
"pair_style gran/history"_pair_style_granular.html - granular potential with history effects
"pair_style gran/no_history"_pair_style_granular.html - granular potential without history effects
"pair_style lj/charmm/coul/charmm"_pair_style_charmm.html - CHARMM potential with cutoff Coulomb
"pair_style lj/charmm/coul/charmm/implicit"_pair_style_charmm.html - CHARMM for implicit solvent
"pair_style lj/charmm/coul/long"_pair_style_charmm.html - CHARMM with long-range Coulomb
"pair_style lj/class2"_pair_style_class2.html - COMPASS (class 2) force field with no Coulomb
"pair_style lj/class2/coul/cut"_pair_style_class2.html - COMPASS with cutoff Coulomb
"pair_style lj/class2/coul/long"_pair_style_class2.html - COMPASS with long-range Coulomb
"pair_style lj/cut"_pair_style_lj.html - cutoff Lennard-Jones potential with no Coulomb
"pair_style lj/cut/coul/cut"_pair_style_lj.html - LJ with cutoff Coulomb
"pair_style lj/cut/coul/debye"_pair_style_lj.html - LJ with Debye damping added to Coulomb
"pair_style lj/cut/coul/long"_pair_style_lj.html - LJ with long-range Coulomb
"pair_style lj/cut/coul/long/tip4p"_pair_style_lj.html - LJ with long-range Coulomb for TIP4P water
"pair_style lj/expand"_pair_style_lj_expand.html - Lennard-Jones for variable size particles
"pair_style lj/smooth"_pair_style_lj_smooth.html - smoothed Lennard-Jones potential
"pair_style morse"_pair_style_morse.html - Morse potential
"pair_style soft"_pair_style_soft.html - Soft (cosine) potential
"pair_style table"_pair_style_table.html - tabulated pair potential
"pair_style yukawa"_pair_style_yukawa.html - Yukawa potential :ul

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[Restrictions:]

This command must come after the simulation box is defined by a
"read_data"_read_data.html, "read_restart"_read_restart.html, or
"create_box"_create_box.html command.

[Related commands:]

"pair_style"_pair_style.html, "pair_modify"_pair_modify.html,
"read_data"_read_data.html, "read_restart"_read_restart.html,
"pair_write"_pair_write.html

[Default:] none