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pair_style command

Syntax:

pair_style style args 

Examples:

pair_style none
pair_style lj/cut 2.5
pair_style lj/cut/coul/cut 2.5
pair_style lj/cut/coul/cut 10.0 8.0
pair_style lj/cut/coul/long 12.0
pair_style lj/expand 2.5
pair_style soft 2.0
pair_style lj/charmm/coul/charmm 10.0 10.3
pair_style lj/charmm/coul/charmm 10.0 10.3 8.0 8.3
pair_style lj/charmm/coul/charmm/implicit 10.0 10.3
pair_style lj/charmm/coul/long 10.0 10.3
pair_style eam
pair_style gran/history 200000.0 0.5 1.0 

pair_style lj/cut/coul/debye 2.5 3.0

Description:

Set the style of interactions for computing pairwise forces and energies. In LAMMPS, pairwise computations include both van der Waals (Lennard Jones) and Coulombic interactions, so there is a range of style choices that encompass the combination of both kinds of forces.

The coefficients for pairwise interactions are set by the pair_coeff command or read from a file by the read_data or read_restart commands. Mixing and shifting of the interaction potentials is discussed with the pair_modify command.

The cutoff arguments set global cutoffs for all type atom pairs which can be overridden by the pair_coeff command. The global cutoff can also be changed by a subsequent pair_style command using the same style. This will only change cutoffs for atom type pairs that have not been explicitly set by the pair_coeff command.

All cutoff arguments are in distance units. The specified cutoff distance(s) can be smaller or larger than the dimensions of the simulation box.

In the formulas to follow, E is the energy of a pairwise interaction between two atoms separated by a distance r. The force between the atoms is the derivative of this expression.

Style none turns off pairwise interactions.

With this choice, the force cutoff is 0.0, which means that only atoms within the neighbor skin distance (see the neighbor command) are communicated between processors. You must insure the skin distance is large enough to acquire atoms needed for computing bonds, angles, etc.

A pair style of none will also prevent pairwise neighbor lists from being built. However if the neighbor style is bin, data structures for binning are still allocated. If the neighbor skin distance is small, then these data structues can consume a large amount of memory. So you should either set the neighbor style to nsq or set the skin distance to a larger value.

Style lj/cut is a cutoff Lennard-Jones (LJ) potential, given by

where epsilon and sigma are coefficients defined for each pair of atom types. Rc is the cutoff.

Style lj/cut/coul/cut computes the same LJ formula as style lj/cut. Additionally, it computes Coulombic pairwise interactions as

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. If one cutoff is specified (in the pair_style or pair_coeff 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.

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

Style lj/expand computes a LJ interaction with a distance shifted by delta

The epsilon, sigma, and delta coefficients are defined for each pair of atom types. Rc is the cutoff.

Style soft is useful for pushing apart overlapping atoms, since it does not blow up as r goes to 0. It computes a pairwise interaction as

where A is a pre-factor that varies in time from the start to the end of the run. Starting and ending values for A are specified by the pair_coeff or read_data command. Rc is the cutoff.

Style lj/charmm/coul/charmm computes LJ and Coulombic interactions with an additional switching function S(r) that ramps the energy and force smoothly to zero between an inner and outer cutoff. It is a widely used option in the CHARMM MD code.

Both the LJ and Coulombic terms require an inner and outer cutoff. They can be the same for both formulas or different depending on whether 2 or 4 arguments are used in the pair_style command. In each case, the inner cutoff distance must be less than the outer cutoff. It it typicaly to make the difference between the 2 cutoffs about 1.0 Angstrom.

Style lj/charmm/coul/charmm/implicit computes the same formulas as style lj/charmm/coul/charmm except that an additional 1/r term is included in the Coulombic formula. The Coulombic energy thus varies as 1/r^2. This is effectively a distance-dependent dielectric term which is a simple model for an implicit solvent with additional screening. It is designed for use in a simulation of an unsolvated biomolecule (no explicit water molecules).

Style lj/charmm/coul/long computes the same formulas as style lj/charmm/coul/charmm except that an additional damping factor is applied to the Coulombic term, as in the discussion for style lj/cut/coul/long.

Style eam computes pairwise interactions for metals and metal alloys using embedded-atom method (EAM) potentials (Daw). The total energy Ei of an atom I is given by

where F is the embedding energy which is a function of the atomic electron density rho, and phi is a pair potential interaction. The multi-body nature of the EAM potential is a result of the embedding energy term. Both summations in the formula are over all neighbors J of atom I within the cutoff distance.

The cutoff distance and the numeric form of F, rho, and phi are listed in one or more files which are specified by the pair_coeff command. Several files for different metals are in the "potentials" directory of the LAMMPS distribution. These are ASCII text files in a DYNAMO-style format. DYNAMO was a serial MD code authored by two of the EAM originators, Stephen Foiles and Murray Daw.

The gran styles use the following formula (Silbert) for frictional force between two granular particles that are a distance r apart when r is less than the contact distance d.

The 1st term is a normal force and the 2nd term is a tangential force. The other quantites are delta = d - r, f(x) = 1 for Hookean contacts used in the pair styles history and no_history, f(x) = sqrt(x) for pair style hertzian, Kn and Kt = elastic constants for normal and tangential contact, gamma_n and gamma_t = viscoelastic constants for normal and tangential contact, m_eff = Mi Mj / (Mi + Mj) = effective mass between 2 particles of mass Mi and Mj , Delta St = the tangential displacement vector between the 2 spherical particles which is truncated to satisfy a frictional yield criterion, n = a unit vector along the line connecting the centers of the 2 particles, and Vn and Vt = normal and tangential components of the relative velocity of the 2 particles. See the citation for more discussion of the granular potentials.

Style lj/charmm/coul/debye computes the same LJ formula as style lj/cut, but the Coulomb formula has an additional term

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

Restrictions:

This command must be used before any coefficients are set by the pair_coeff, read_data, or read_restart commands.

Some pair styles are part of specific packages. They are only enabled if LAMMPS was built with that package. See the Making LAMMPS section for more info.

The gran/hertzian, gran/history, and gran/no_history styles are part of the "granular" package. The lj/charmm/coul/charmm and lj/charmm/coul/charmm/implicit styles are part of the "molecule" package. The lj/cut/coul/long and lj/charmm/coul/long styles are part of the "kspace" package. The eam) style is part of the "metal" package.

Related commands:

pair_coeff, read_data, pair_modify, kspace_style, dielectric

Default:

pair_style none 

(Daw) Daw, Baskes, Phys Rev Lett, 50, 1285 (1983). Daw, Baskes, Phys Rev B, 29, 6443 (1984).

(Silbert) Silbert, Ertas, Grest, Halsey, Levine, Plimpton, Phys Rev E, 64, p 051302 (2001).