Abstract
A perturbation theory is developed for the correlation energy [n], of a finite-density system, with respect to the coupling constant α which multiplies the electron-electron repulsion operator in =T^+αV+(). The external potential is constrained to keep the gound-state density n fixed for all α≥0. is given completely in terms of functional derivatives at full charge (α=1), from which []=[n]+ [n]+[n]+..., where each j[n] is expressed in terms of integrals involving Kohn-Sham determinants. Here, (x,y,x)=n(λx,λy,λz) and λ=. The identification of [], which is a high-density limit, as the second-order energy [n] allows one to compute bounds upon []; the bounds imply that []≃[n] for a large class of small atoms and molecules, and suggest that [] should be of the same order of magnitude as [n] in finite insulators and semiconductors.
Approximations to [n] should reflect all this. In contrast, perhaps the well-known overbinding of the local-density approximation (LDA) in molecules and solids is due, in part, to the fact that the LDA correlation energy is too sensitive to a coordinate scaling of n. Indeed, the LDA for [] diverges when λ→∞ because of the presence of the -ln(λ) term in the Gell-Mann and Brueckner high-density expression for the correlation energy, per particle, of a homogeneous density, which is infinite. In a sense, the derived perturbation expansion transforms the Gell-Mann and Brueckner expression into one that applies specifically to an inhomogeneous density which integrates to a finite number of electrons.
- Received 30 November 1992
DOI:https://doi.org/10.1103/PhysRevB.47.13105
©1993 American Physical Society