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

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( and CH2. 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 Iii Pad4 approximants and deviations of solutions by the present method from the strict size consistency are discussed.