Order-by-order intermediate Hamiltonian expansions: applications to ab initio molecular calculations

(Received 9 June 1993; accepted 23 July 1993)

Abstract
A simple third-order intermediate Hamiltonian method based on the level shift technique is presented and applied to ab initio calculations on low-lying electronic states of CH ⁺, CH ₂⁺ and CH ₂. The method is shown to provide a good approximation to full CI for both total energies and excitation energies. Numerous intruder states did not give rise to any instability problems. Series resummation using modified operator [2/1] Padé approximants and deviations of solutions by the present method from the strict size consistency are discussed.