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 det ermined S-n crystal structures are used to define a new spherical S ... S i ntermolecular force field of the (exp-6) type. Evidence is presented that t he bonded sulfur atom in these structures is not spherical. The asphericity was modeled by placing repulsion bumps at optimized locations on the sulfu r atoms to represent lone pair electron repulsion. The bump locations devia te from perfect tetrahedral geometry, being located more perpendicular to t he S-S-S plane. A new aspherical sulfur intermolecular force field was deri ved from eleven S-n crystal structures. The aspherical force field gives an improved prediction of the S-n crystal structures.