Aspects of density functional theory in ab initio quantum chemistry: external correlation for free

The restricted Hartree-Fock method is subject to a first-order perturbation theory regularization of the electron repulsion integrals representation, to mimic the electron-electron correlation hole effect. The efficiency of this regularization is demonstrated for electron affinities and ionization energies of atoms, and on atomization energies of molecules. The accuracy of the method is comparable to that of modern density functional approaches as it provides significant improvement on standard Hartree-Fock results. Conceptual similarities with density functional theory are discussed, as are the implications for ab initio calculations on large systems.