Density-functional formalism and the two-body problem

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Abstract
Some aspects related to the behavior of the correlation energy as a functional of the density are not well represented by the density functionals; such is the case of the energy curves for pure states of diatomic molecules, the dependence of the correlation energy on the zeroth-order wave function, or the multiplet splitting in atoms. In this paper we show that these limitations are due to the fact that a complete description of the two-body effects requires one to take into account the two-electron density. It is shown that using very simple approximations to incorporate explicitly the two-body density into the density-functional equations improves their behavior considerably.