(Received 31 July 1997)
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.
PACS: 34.30.+h, 31.15.Ew, 71.15.Mb
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