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