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### pair_style buck/coul command

Syntax:

    pair_style buck/coul flag_buck flag_coul cutoff (cutoff2)

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

Examples:

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

Description:

The buck/coul style computes a Buckingham potential (exp/6 instead of Lennard-Jones 12/6) and Coulombic potential, given by

Rc is the cutoff. If one cutoff is specified in the pair_style command, it is used for both the Buckingham and Coulombic terms. If two cutoffs are specified, they are used as cutoffs for the Buckingham and Coulombic terms respectively.

The purpose of this pair style is to capture long-range interactions resulting from both attractive 1/r^6 Buckingham and Coulombic 1/r interactions. This is done by use of the flag_lj and flag_coul settings. The "Ismail 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 Buckingham 1/r^6 dispersion term. The long-range portion is calculated by using the kspace_style ewald/n command. The specified Buckingham cutoff then determines which portion of the Buckingham 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 Buckingham interactions are simply cutoff, as with pair_style buck.

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_buck is also set to long, then only the ewald/n Kspace style can perform the long-range calculations for both the Buckingham 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:

- A (energy units)
- rho (distance units)
- C (energy-distance^6 units)
- cutoff (distance units)
- cutoff2 (distance units)

The second coefficient, rho, must be greater than zero.

The latter 2 coefficients are optional. If not specified, the global Buckingham 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 Buckingham and Coulombic interactions for this type pair. If both coefficients are specified, they are used as the Buckingham and Coulombic cutoffs for this type pair. Note that if you are using flag_buck set to long, you cannot specify a Buckingham cutoff for an atom type pair, since only one global Buckingham 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:

This pair styles does not support mixing. Thus, coefficients for all I,J pairs must be specified explicitly.

This pair style supports the pair_modify shift option for the energy of the exp() and 1/r^6 portion of the pair interaction, assuming flag_buck is cut.

This pair style does not support the pair_modify shift option for the energy of the Buckingham portion of the pair interaction.

This pair style does not support the pair_modify table option since a tabulation capability has not yet been added to this potential.

This pair style write 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.

Related commands:

pair_coeff

Default: none

(Ismail) Ismail, Tsige, In 't Veld, Grest, Molecular Physics (accepted) (2007).