  We start from the Bethe-Goldstone equation (BGE) to derive a simple
orbital-dependent correlation functional -- BGE2 -- which terminates the BGE
expansion at the second-order, but retains the self-consistent coupling of
electron-pair orrelations. We demonstrate that BGE2 is size consistent and
one-electron "self-correlation" free. The electron-pair correlation coupling
ensures the correct H$_2$ dissociation limit and gives a finite correlation
energy for any system even if it has a no energy gap. BGE2 provides a good
description of both H$_2$ and H$_2^+$ dissociation, which is regarded as a
great challenge in density functional theory (DFT). We illustrate the behavior
of BGE2 analytically by considering H$_2$ in a minimal basis. Our analysis
shows that BGE2 captures essential features of the adiabatic connection path
that current state-of-the-art DFT approximations do not.
