Quasi-local-density approximation for a van der Waals energy functional

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We discuss a possible form for a theory akin to local density functional theory, but able to produce van der Waals energies in a natural fashion. The usual Local Density Approximation (LDA) for the exchange and correlation energy Exc of an inhomogeneous electronic system can be derived by making a quasilocal approximation for the interacting density-density response function χ(r,r ′,ω), then using the fluctuation-dissipation theorem and a Feynman coupling-constant integration to generate Exc. The first new idea proposed here is to use the same approach except that one makes a quasilocal approximation for the bare response χ⁰, rather than for χ. The interacting response is then obtained by solving a nonlocal screening integral equation in real space. If the nonlocal screening is done at the time-dependent Hartree level, then the resulting energy is an approximation to the full inhomogeneous RPA energy: we show here that the inhomogeneous RPA correlation energy contains a van der Waals term for the case of widely-separated neutral subsystems. The second new idea is to use a particularly simple way of introducing LDA-like local field corrrections into the screening equations, giving a theory which should remain reasonable for all separations of a pair of subsystems, encompassing both the van der Waals limit much as in RPA and the bonding limit much as in LDA theory.