Prospects for a Van der Waals Density Functional

Abstract

The van der Waals (vdW) or dispersion interaction between distant sections of a molecular or condensed matter system contributes part of the correlation energy. This dispersion part is missed, or is given at best unreliably, by the usual local and gradient functionals for the exchange–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 proposed is “seamless,” i.e., it remains valid whether or not the subsystems involved have significant overlap of electronic density. Perturbative expansion of this scheme, followed by a doubly local density approximation, 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 overlap between the electron clouds. Progress is reported on the full seamless scheme which deals with the more challenging case where overlap is present. All of the present schemes yield the classic dipolar “van der Waals” 1/R6 energy when the separation R sufficiently exceeds the individual system dimensions. None is restricted to this case, however.