Variational configuration interaction methods and comparison with perturbation theory

A configuration interaction (Cl) procedure which includes all single and double substitutions from an unrestricted Hartree-Fock single determinant is described. This has the feature that Møller-Plesset perturbation results to second and third order are obtained in the first Cl iterative cycle. The procedure also avoids the necessity of a full two-electron integral transformation. A simple expression for correcting the final Cl energy for lack of size consistency is proposed. Finally, calculations on a series of small molecules are presented to compare these Cl methods with perturbation theory.