Spherical and aspherical intermolecular force fields for sulfur allotropes

Elemental sulfur exists in many crystalline forms, with many of these forms being composed of discrete S-n cyclic molecules. Eleven experimentally determined S-n crystal structures are used to define a new spherical S ... S intermolecular force field of the (exp-6) type. Evidence is presented that the bonded sulfur atom in these structures is not spherical. The asphericity was modeled by placing repulsion bumps at optimized locations on the sulfur atoms to represent lone pair electron repulsion. The bump locations deviate from perfect tetrahedral geometry, being located more perpendicular to the S-S-S plane. A new aspherical sulfur intermolecular force field was derived from eleven S-n crystal structures. The aspherical force field gives an improved prediction of the S-n crystal structures.