Calculation of dispersion energy shifts in molecular electronic spectra using sum rules

The use of factorization formulas due to Casimir and Polder gives exact expressions for van der Waals energies in terms of the dynamic properties of the subsystems. It is natural to approximate the appearing polarizabilities in terms of rational fractions involving moments of the oscillator strength distributions as previously derived by the present author. The validity of sum rules justifies the procedure. In this work results using the simplest Fade approximants are presented for the dispersion energy associated to two subsystems with the goal of comparing with results of Zerner and co-workers for a solute in the presence of a solvent. (C) 2001 John Wiley & Sons, Inc.