  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 inter-system distances. The asymptotic H-He and He--He
interactions are calculated as a first illustration, with very accurate
results.
