Are direct reaction field methods appropriate for describing dispersion interactions?

Abstract

Direct reaction field (DRF) model Hamiltonians are used in various fields of quantum chemistry, for example in core polarization potentials and in solvent effect theories. The DRF method is usually claimed to be appropriate to describe the correlation (dispersion interaction) with other electron groups, which are not included explicitly in the calculation. It is shown that the C 6 coefficients resulting from the DRF theory are erroneous by a factor of 2 to 15 in a series of simple systems. The source of the error can be attributed to the unjustified assumption that the mean excitation energy of the explicitly treated subsystem is negligible with respect to the excitation energies of the other one. A more appropriate reaction field Hamiltonian is proposed which includes both “average” (nonlinear) and “direct” (linear) terms for polar solutes.