Intermolecular Forces in Molecular Crystals.

In the present work the nature of intermolecular forces is examined thoroughly. One of the major conclusions is that the bonding electrons, largely ignored or dismissed in other treatments, have to be included in the description of intermolecular forces. We have used, for the first time, anisotropic dispersion forces between polarizable centers other than between atoms, namely bond-bond and atom-bond, to study the stability of the crystal structures of F_2, Cl_2 and C_2H _2.. The Model used here is the Supermolecule Method calculated at the Hartree-Fock level and supplemented with the Dispersion energy. The latter is expressed as a sum of interactions between localized molecular orbitals and the molecular polarizability is partitioned into its natural components, namely bond, atomic and lone pair electron contributions. Considerable success has been achieved in predicting the space group of small molecules. This indicates that we have improved our understanding on intermolecular forces in molecular crystals. Future directions to improve further the present description of intermolecular forces are discussed.