pair_style gayberne command

Syntax:

pair_style gayberne gamma upsilon mu cutoff 

• gamma = shift for potential minimum (typically 1)
• upsilon = exponent for eta orientation-dependent energy function
• mu = exponent for chi orientation-dependent energy function
• cutoff = global cutoff for interactions (distance units)

Examples:

pair_style gayberne 1.0 1.0 1.0 10.0
pair_coeff * * 1.0 1.7 1.7 3.4 3.4 1.0 1.0 1.0 

Description:

Style gayberne computes a Gay-Berne anisotropic LJ interaction (Beradi) between pairs of ellipsoidal particles via the formulas

where A1 and A2 are the transformation matrices from the simulation box frame to the body frame and r12 is the center to center vector between the particles. Ur controls the shifted distance dependent interaction based on the distance of closest approach of the two particles (h12) and the user-specified shift parameter gamma.

For large uniform molecules it has been shown that the energy parameters are approximately representable in terms of local contact curvatures (Everaers):

The variable names utilized as potential parameters are for the most part taken from (Everaers) in order to be consistent with its RE-squared potential fix. Details on the upsilon and mu parameters are given here. Use of this pair style requires the fix nve/asphere in order to integrate particle rotation. Additionally, atom_style ellipsoid should be used since it defines the rotation state of the ellipsoidal particles.

More details of the Gay-Berne formulation are given in the references listed below and in this document.

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:

• epsilon = well depth (energy units)
• sigma = minimum effective particle radii (distance units)
• a = ellipsoid radius in x dimension (distance units)
• b = ellipsoid radius in y dimension (distance units)
• c = ellipsoid radius in z dimension (distance units)
• epsilon_a = relative well depth for side-to-side interactions
• epsilon_b = relative well depth for face-to-face interactions
• epsilon_c = relative well depth for end-to-end interactions
• cutoff (distance units)

The last coefficient is optional. If not specified, the global cutoff specified in the pair_style command is used.

The epsilon and sigma parameters are mixed for I != J atom pairings the same as Lennard-Jones parameters; see the pair_modify mix documentation for details. The other parameters (except cutoff) are really specific to a single atom type, and not a pair of atoms. Thus they are applied to atom type I only.

Restrictions:

Can only be used if LAMMPS was built with the "asphere" package. Can only be used with atom_style ellipsoid.

The use of this potential requires additional fixes as described above. The "shift yes" option currently cannot be used with this potential to shift energies to 0 at the cutoff due to the anisotropic dependence of the interaction. Angular velocities are all set to zero initially. The Gay-Berne potential does not become isotropic as r increases (Everaers). The distance of closest approach approximation becomes less accurate as the shape of ellipsoids becomes more dissimilar (high aspect ratio particles).

Related commands:

pair_coeff, fix nve/asphere, compute temp/asphere

Default: none

(Everaers) Everaers and Ejtehadi, Phys Rev E, 67, 041710 (2003).

(Berardi) Berardi, Fava, Zannoni, Chem Phys Lett, 297, 8-14 (1998).

(Perram) Perram and Rasmussen, Phys Rev E, 54, 6565-6572 (1996).

(Allen) Allen and Germano, Mol Phys 104, 3225-3235 (2006).