Comments on the Basis Sets used in Recent Studies of Electron Correlation in Small Molecules

Abstract

Some comments are made on the basis sets employed in recent studies of electron correlation energies for small molecules with particular reference to calculations for the ground states of the nitrogen and water molecules. For diatomic systems, the use of finite difference and finite basis set approximations in generating the Hartree-Fock reference function is compared. The distributed, universal, even-tempered basis sets (for which we introduce the acronym duet) and correlation consistent basis sets are compared for both the Hartree-Fock model and in treatments of correlation effects. The use of correlation treatments based on many-body perturbation theory and on coupled cluster expansions are discussed. The systematic approximation of the molecular integral supermatrix corresponding to duet basis sets is addressed, as are applications to molecular systems containing heavy atoms.