Van der Waals forces in the local-orbital density functional theory

Van der Waals forces are analyzed in a Density Functional Theory, using a "local orbital occupancy" formulation and second-order perturbation theory. In this approach, the exchange-correlation energy as well as the van der Waals forces are written as a function of the orbital occupation numbers. We present a detailed discussion of the He-He case and calculate the Density Functional-van der Waals energy of the system. Our analysis also suggests an alternative approach for including van der Waals forces in the local-orbital DFT formulation, namely, to introduce an effective hopping interaction between the orbitals of both atoms. Our results for the He and Ne dimers show the validity and the accuracy of our proposed DFT-van der Waals approach.