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### pair_style coul/cut command

### pair_style coul/debye command

### pair_style coul/long command

Syntax:

    pair_style coul/cut cutoff
    pair_style coul/debye kappa cutoff
    pair_style coul/long cutoff

- cutoff = global cutoff for Coulombic interactions
- kappa = Debye length (inverse distance units)

Examples:

    pair_style coul/cut 2.5
    pair_coeff * *
    pair_coeff 2 2 3.5

    pair_style coul/debye 1.4 3.0
    pair_coeff * *
    pair_coeff 2 2 3.5

    pair_style coul/long 10.0
    pair_coeff * *

Description:

The coul/cut style computes the standard Coulombic interaction potential given by

where C is an energy-conversion constant, Qi and Qj are the charges on the 2 atoms, and epsilon is the dielectric constant which can be set by the dielectric command. The cutoff Rc truncates the interaction distance.

Style coul/debye adds an additional exp() damping factor to the Coulombic term, given by

where kappa is the Debye length. This potential is another way to mimic the screening effect of a polar solvent.

Style coul/long computes the same Coulombic interactions as style coul/cut except that an additional damping factor is applied so it can be used in conjunction with the kspace_style command and its ewald or pppm option. The Coulombic cutoff specified for this style means that pairwise interactions within this distance are computed directly; interactions outside that distance are computed in reciprocal space.

These potentials are designed to be combined with other pair potentials via the pair_style hybrid/overlay command. This is because they have no repulsive core. Hence if they are used by themselves, there will be no repulsion to keep two oppositely charged particles from overlapping each other.

The following coefficients must be defined for each pair of atoms types via the pair_coeff command as in the examples above, or in the data file or restart files read by the read_data or read_restart commands, or by mixing as described below:

- cutoff (distance units)

For coul/cut and coul/debye, the cutoff coefficient is optional. If it is not used (as in some of the examples above), the default global value specified in the pair_style command is used.

For coul/long no cutoff can be specified for an individual I,J type pair via the pair_coeff command. All type pairs use the same global Coulombic cutoff specified in the pair_style command.

Mixing, shift, table, tail correction, restart, rRESPA info:

For atom type pairs I,J and I != J, the cutoff distance for the coul/cut style can be mixed. The default mix value is geometric. See the "pair_modify" command for details.

The pair_modify shift option is not relevant for these pair styles.

The coul/long style supports the pair_modify table option for tabulation of the short-range portion of the long-range Coulombic interaction.

These pair styles do not support the pair_modify tail option for adding long-range tail corrections to energy and pressure.

These pair styles write their information to binary restart files, so pair_style and pair_coeff commands do not need to be specified in an input script that reads a restart file.

This pair style can only be used via the pair keyword of the run_style respa command. It does not support the inner, middle, outer keywords.

Restrictions:

The coul/long style is part of the "kspace" package. It is only enabled if LAMMPS was built with that package (which it is by default). See the Making LAMMPS section for more info.

On some 64-bit machines, compiling with -O3 appears to break the Coulombic tabling option used by the coul/long style. See the "Additional build tips" section of the Making LAMMPS documentation pages for workarounds on this issue.

Related commands:

pair_coeff, pair_style hybrid/overlay

Default: none