Electron pairs, localized orbitals and electron correlation

Presuming zeroth-order electronic wavefunctions generated from localized SCF or FORS molecular orbitals, the correlation energy is expressed as a bilinear form in terms of the pair populations of these orbitals and the projections of a correlation operator onto these orbitals. The latter are determined by fitting the correlation energies of large sets of organic molecules, which are reproduced with a mean absolute deviation of 1-3 kcal mol(1). The resulting formula provides a `back-of-the-envelope' method for estimating correlation energies and furnishes an analysis of these energies in terms of physical concepts and chemical structure. It predicts the correlation energy of diamond (per carbon atom) to within 6 kcal mol(1).