Pseudonatural Orbitals as a Basis for the Superposition of Configurations. I. He2+

Abstract

The use of pseudonatural orbitals (PNO) is proposed to improve the rate of convergence in the superposition of configurations (SOC). Natural orbitals are determined for selected electron pairs in the Hartree—Fock field of the n−2 electron core and are then used as the basis for the total SOC calculation. Since these natural orbitals are not natural for the n‐electron system they are considered false or pseudonatural orbitals when used in the n‐electron problem. The PNO basis has been applied to He₂⁺ and H₃ to test the convergence. Complete results are reported here only for He₂⁺. The PNO's are quite successful in speeding up the convergence of the SOC and rendering the calculation of correlation energy quite practical in general. Gaussian‐type orbitals (GTO) are used throughout and were not a serious impediment to obtaining quantitative accuracy. In fact the large number of unoccupied Hartree—Fock orbitals consequent upon the use of a GTO basis permit a straightforward determination of the PNO orbitals.