CALCULATION OF DISPERSION ENERGY SHIFTS IN MOLECULAR ELECTRONIC-SPECTRA

An efficient quantum chemical method for calculating dispersion energy shifts in molecular electronic spectra is described. The method makes use of separate calculations for solvent and solute molecules using a perturbation theory formula which averages over solvent configuration s. To maintain the balance between the dispersion depression of the gr ound state and that of an excited state in a finite configuration inte raction (CI) treatment, the Thomas-Reiche-Kuhn sum rule is invoked twi ce, correcting for the missing higher excited and continuum states in both the solute and the solvent. The resulting modified perturbation e xpression for the dispersion energy is remarkably stable with respect to the size of the employed CI. The model is implemented and applied i n the framework of the intermediate neglect of diatomic overlap method using a previously presented self-consistent reaction field descripti on for the solvent and the solute molecules. Applications to acetone, benzene, naphthalene, and chrysene in cyclohexane clearly demonstrate the success of the method.