van der Waals Energies in Density Functional Theory

In principle, density functional theory yields the correct ground-state densities and energies of electronic systems under the action of a static external potential. However, traditional approximations fail to include van der Waals energies between separated systems. This paper proposes a practical procedure for remedying this difficulty. Our method allows seamless calculations between small and large intersystem distances. The asymptotic H-He and He--He interactions are calculated as a first illustration, with very accurate results.