Hierarchies of intermolecular potentials and forces: Progress towards a quantitative evaluation

Conventional views of intermolecular cohesion, based on the traditional categories of hydrogen bonding, of aromatic interactions, of dipolar or quadrupolar contacts, and of the broad, gray zone of 'van der Waals' liaison, often define strength hierarchies on the basis of qualitative categories like approximate molecular orientations or distances between atomic nuclei in molecules. When interaction energies are quantitatively evaluated between molecular pairs, in a more justifiable partitioning scheme, often a completely different picture emerges. Examples are given for selected molecular dimers and organic crystals, using a new semiempirical scheme, the PIXEL method, which also allows a separate evaluation of coulombic, polarization, dispersion and repulsion energy terms.