ORDER-BY-ORDER INTERMEDIATE HAMILTONIAN EXPANSIONS - APPLICATIONS TO AB-INITIO MOLECULAR CALCULATIONS

A simple third-order intermediate Hamiltonian method based on the leve l shift technique is presented and applied to ab initio calculations o n low-lying electronic states of CH+ CH2+ and CH2. The method is shown to provide a good approximation to full CI for both total energies an d excitation energies. Numerous intruder states did not give rise to a ny instability problems. Series resummation using modified operator [2 /1] Pade approximants and deviations of solutions by the present metho d from the strict size consistency are discussed.