Weak Interactions Between Molecules

Explicit formulae are derived for the calculation of dispersion energies between large molecules, at various levels of approximmion. The derivation introduces frequency-dependent polarizabilities (FDPs), whiCh describe the propagation of electron density fluctuat· ions within each of the separate molecules, but avoids the usual multipole expansion. The resultant dispersion energy formula reveals the presence of long-range (R-2) energy terms between moms of the different molecules and provides a basis for semi-empirical models based on pairwise atom-atom interactions. A rapidly convergent SCF procedure for calculating the required FDPs is also described.