DISPERSION INTERACTION BETWEEN ELONGATED MOLECULES

As has been recognized for some time, the dispersion interaction of mo lecules, particularly large molecules, cannot be adequately represente d by a simple London-type expression based on the total molecule polar izabilities. We investigate two models consisting of several anisotrop ic polarizabilities associated with molecular subgroups. In one model intramolecular interactions among the groups are included; in the othe r only intermolecular interactions are included, but different ''effec tive'' polarizabilities are used. These models are generalizations of models considered previously by others. We calculate the interaction e nergy as a function of molecular separation for several different rela tive orientations of the molecular axes of model PAA (p-azoxyanisole) molecules. We find that (1) at very short distances intramolecular int eraction is negligible; (2) at intermediate distances the distance dep endence is approximately inverse fifth power; (3) the two models yield almost identical results over all but the very shortest distances; (4 ) at molecular separations greater than a few times the largest molecu lar dimension, the distance dependence is essentially the simple inver se sixth power London interaction based on total molecular polarizabil ity. We compare these qualitative results with those found previously by others.