The use of factorization formulas due to Casimir and Polder gives exact exp
ressions for van der Waals energies in terms of the dynamic properties of t
he 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 tw
o subsystems with the goal of comparing with results of Zerner and co-worke
rs for a solute in the presence of a solvent. (C) 2001 John Wiley & Sons, I
nc.