Adiabatic connection at negative coupling strengths

The adiabatic connection of density functional theory (DFT) for electronic systems is generalized here to negative values of the coupling strength $α$ (with {\em attractive} electrons). In the extreme limit $α\to-\infty$ a simple physical solution is presented and its implications for DFT (as well as its limitations) are discussed. For two-electron systems (a case in which the present solution can be calculated exactly), we find that an interpolation between the limit $α\to-\infty$ and the opposite limit of infinitely strong repulsion ($α\to+\infty$) yields a rather accurate estimate of the second-order correlation energy $E\cor\glt[ρ]$ for several different densities $ρ$, without using virtual orbitals. The same procedure is also applied to the Be isoelectronic series, analyzing the effects of near-degeneracy.

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