Density Functionals for Correlation Energies of Atoms and Molecules

Abstract

The correlation energy, Ec, is usually defined as the difference of the exact (non-relativistic) energy, E, and the Hartree-Fock (HF) energy, EHF .1 Ec is a very small part of E only (1.4% for the He atom, 0.3% for Ne, 0.1% for Ar), but it is non-negligible in absolute value: for valence-shell removal, ...Ec is 1.1 eV for He, 9.5 eV for Ne, and 9.3 eV for Ar. Inclusion of Ec is important in cases where the number of (strongly interacting) electron pairs is changed, for dissociation energies (De), ionization potentials and excitation energies, e.g.. Correlation is responsible for 23% of Dein the case of H2, and for 84% of De in the case of Li2; Na2 and K2 are unbound at the HF level.