Syntax:
pair_style dipole/cut cutoff (cutoff2)
Examples:
pair_style dipole/cut 10.0 pair_coeff * * 1.0 1.0 pair_coeff 2 3 1.0 1.0 2.5 4.0
Description:
Style dipole/cut computes interactions bewteen pairs of particles that each have a charge and/or a point dipole moment. In addition to the usual Lennard-Jones interaction between the particles (Elj) the charge-charge (Eqq), charge-dipole (Eqp), and dipole-dipole (Epp) interactions are computed by these formulas for the energy (E), force (F), and torque (T) between particles I and J.
where qi and qj are the charges on the two particles, pi and pj are the dipole moment vectors of the two particles, r is their separation distance, and the vector r = Ri - Rj is the separation vector between the two particles. Note that Eqq and Fqq are simply Coulombic energy and force, Fij = -Fji as symmetric forces, and Tij != -Tji since the torques do not act symmetrically. These formulas are discussed in (Allen) and in (Toukmaji).
If one cutoff is specified in the pair_style command, it is used for both the LJ and Coulombic (q,p) terms. If two cutoffs are specified, they are used as cutoffs for the LJ and Coulombic (q,p) terms respectively.
Use of this pair style requires the use of the fix nve/dipole command to integrate rotation of the dipole moments. Additionally, atom_style dipole should be used since it defines the point dipoles and their rotational state. The magnitude of the dipole moment for each type of particle can be defined by the dipole command or in the "Dipoles" section of the data file read in by the read_data command. Their initial orientation can be defined by the set dipole command or in the "Atoms" section of the data file.
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:
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.
Restrictions:
Can only be used if LAMMPS was built with the "dipole" package.
The use of this potential requires additional fixes as described above.
Related commands:
pair_coeff, fix nve/dipole, compute temp/dipole
Default: none
(Allen) Allen and Tildesley, Computer Simulation of Liquids, Clarendon Press, Oxford, 1987.
(Toukmaji) Toukmaji, Sagui, Board, and Darden, J Chem Phys, 113, 10913 (2000).