COMPUTATION OF CHARGE-TRANSFER ENERGIES BY PERTURBATION-THEORY

The charge-transfer energy of a van der Waals complex is an ill-defined part of the induction (polarization) energy, and vanishes in the limit of a complete basis. However in the case of a finite basis the two effects can be distinguished, although the distinction is basis-set dependent and to some extent arbitrary, and the charge-transfer term can be computed in a BSSE-free manner. In the cases examined the charge-transfer energy is small, though not always negligible: for instance there is a contribution to the binding energy of the water dimer that is approximately -4 kJ mol-1 at the equilibrium geometry.