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 configurations. To maintain the balance between the dispersion depression of the ground state and that of an excited state in a finite configuration interaction (CI) treatment, the Thomas-Reiche-Kuhn sum rule is invoked twice, correcting for the missing higher excited and continuum states in both the solute and the solvent. The resulting modified perturbation expression for the dispersion energy is remarkably stable with respect to the size of the employed CI. The model is implemented and applied in the framework of the intermediate neglect of diatomic overlap method using a previously presented self-consistent reaction field description for the solvent and the solute molecules. Applications to acetone, benzene, naphthalene, and chrysene in cyclohexane clearly demonstrate the success of the method.