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### pair_style lj/coul command

Syntax:

    pair_style lj/coul flag_lj flag_coul cutoff (cutoff2)

- flag_lj = long or cut

        long = use Kspace long-range summation for the dispersion term 1/r^6
        cut = use a cutoff

- flag_coul = long or off

        long = use Kspace long-range summation for the Coulombic term 1/r
        off = omit the Coulombic term

- cutoff = global cutoff for LJ (and Coulombic if only 1 cutoff) (distance units)
- cutoff2 = global cutoff for Coulombic (optional) (distance units)

Examples:

    pair_style lj/coul cut off 2.5
    pair_style lj/coul cut long 2.5 4.0
    pair_style lj/coul long long 2.5 4.0
    pair_coeff * * 1 1
    pair_coeff 1 1 1 3 4

Description:

The lj/coul style computes the standard 12/6 Lennard-Jones and Coulombic potentials, given by

where C is an energy-conversion constant, Qi and Qj are the charges on the 2 atoms, epsilon is the dielectric constant which can be set by the dielectric command, and Rc is the cutoff. If one cutoff is specified in the pair_style command, it is used for both the LJ and Coulombic terms. If two cutoffs are specified, they are used as cutoffs for the LJ and Coulombic terms respectively.

The purpose of this pair style is to capture long-range interactions resulting from both attractive 1/r^6 Lennard-Jones and Coulombic 1/r interactions. This is done by use of the flag_lj and flag_coul settings. The In 't Veld paper has more details on when it is appropriate to include long-range 1/r^6 interactions, using this potential.

If flag_lj is set to long, no cutoff is used on the LJ 1/r^6 dispersion term. The long-range portion is calculated by using the kspace_style ewald/n command. The specified LJ cutoff then determines which portion of the LJ interactions are computed directly by the pair potential versus which part is computed in reciprocal space via the Kspace style. If flag_lj is set to cut, the LJ interactions are simply cutoff, as with pair_style lj/cut.

If flag_coul is set to long, no cutoff is used on the Coulombic interactions. The long-range portion is calculated by using any style, including ewald/n of the kspace_style command. Note that if flag_lj is also set to long, then only the ewald/n Kspace style can perform the long-range calculations for both the LJ and Coulombic interactions. If flag_coul is set to off, Coulombic interactions are not computed.

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:

- epsilon (energy units)
- sigma (distance units)
- cutoff1 (distance units)
- cutoff2 (distance units)

Note that sigma is defined in the LJ formula as the zero-crossing distance for the potential, not as the energy minimum at 2^(1/6) sigma.

The latter 2 coefficients are optional. If not specified, the global LJ and Coulombic cutoffs specified in the pair_style command are used. If only one cutoff is specified, it is used as the cutoff for both LJ and Coulombic interactions for this type pair. If both coefficients are specified, they are used as the LJ and Coulombic cutoffs for this type pair. Note that if you are using flag_lj set to long, you cannot specify a LJ cutoff for an atom type pair, since only one global LJ cutoff is allowed. Similarly, if you are using flag_coul set to long, you cannot specify a Coulombic cutoff for an atom type pair, since only one global Coulombic cutoff is allowed.

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

For atom type pairs I,J and I != J, the epsilon and sigma coefficients and cutoff distance for all of the lj/cut pair styles can be mixed. The default mix value is geometric. See the "pair_modify" command for details.

This pair style supports the pair_modify shift option for the energy of the Lennard-Jones portion of the pair interaction, assuming flag_lj is cut.

This pair style supports the pair_modify table option since it can tabulate the short-range portion of the long-range Coulombic interaction.

This pair styles supports the pair_modify tail option for adding a long-range tail correction to the Lennard-Jones portion of the energy and pressure of the pair interaction, assuming flag_lj is cut.

This pair style writes its 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 supports the use of the inner, middle, and outer keywords of the run_style respa command, meaning the pairwise forces can be partitioned by distance at different levels of the rRESPA hierarchy. See the run_style command for details.

Restrictions:

This style is part of the "user-ewaldn" package. It is only enabled if LAMMPS was built with that package. 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 lj/coul style. See the "Additional build tips" section of the Making LAMMPS documentation pages for workarounds on this issue.

Related commands:

pair_coeff

Default: none

(In 't Veld) In 't Veld, Ismail, Grest, J Chem Phys (accepted) (2007).