On the application of MCSCF models for the calculation of spectroscopic constants for several low lying states of O2+

Abstract

The behavior of multiconfiguration self-consistent field (MCSCF) and restricted configuration interaction (CI) wave functions are investigated for the case of a homonuclear positive diatomic, i.e., O2+. Both molecular orbital (MO) and orthogonalized valence bond (VB) models are used. In the MO model the normal symmetry restrictions (D∞h) are imposed on the orbital while these are lifted in the VB model, thus permitting a more correct behavior on dissociation. Using He2+ as a prototype system, the customary partitioning of the MO configuration set into subsets corresponding to proper dissociation, molecular extra correlation, and atomic correlation is examined. It is shown that when the orbitals are optimized this partitioning is no longer valid, i.e., atomic correlation cannot be excluded. For O2+ the proper dissociation function in the MO model is rather unwieldy. This arises because allowance must be made for the difference between the atomic orbitals of O and O+. We therefore use, as our starting point in the configuration selection procedure, the much simpler base wave function which corresponds to a correct dissociation limit in a minimal basis set. The VB model does not suffer from these difficulties. However, due to the absence of symmetry restrictions, there is a symmetry breakdown in the VB calculation in the range 1.5<R<2.5 Å caused by a transition from D∞h symmetry in the equilibrium conformation to effective C∞v symmetry for R = ∞. Using a double zeta + polarization basis, satisfactory results were obtained for the spectroscopic constants of four of the five lowest lying states. viz., 2Πg, 4Πu, 2Πu, and 2Φu, via MO–MCSCF calculations using the base wave function augmented by all valence shell (2p) configurations of the correct symmetry. For the 4Πu and for the 4Σg− state (for which poor results were obtained in the MO model) VB–CI calculations were also performed. In these calculations all single excitations from a reference state consisting of the proper dissociation function and all valence shell (2p) C∞v configurations were included, leading to satisfactory agreement with experimental data.