PROSPECTS FOR A VAN-DER-WAALS DENSITY-FUNCTIONAL

The van der Waals (vdW) or dispersion interaction between distant sect ions of a molecular or condensed matter system contributes part of the correlation energy. This dispersion part is missed, or is given at be st unreliably, by the usual local and gradient functionals for the exc hange-correlation energy. Here we discuss various quasi-local schemes which are expected to provide good vdW energies using only a knowledge of the ground-state electron density n(r). The general scheme propose d is ''seamless,'' i.e., if remains valid whether or not the subsystem s involved have significant overlap of electronic density. Perturbativ e expansion of this scheme, followed by a doubly local density approxi mation, yields a simple vdW scheme proposed independently by Andersson et al., and by Dobson and Dinte, which is also similar to a scheme of Rapcewicz and Ashcroft. These simpler schemes cannot deal with overla p between the electron clouds. Progress is reported on the full seamle ss scheme which deals with the more:challenging case where overlap is present. All of the present schemes yield the classic dipolar ''van de r Waals'' 1/R-6 energy when the separation R sufficiently exceeds the individual system dimensions. None is restricted to this case, however . (C) 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 615-618, 1998.