COMPARISON OF HIGH-ORDER PERTURBATIVE CONVERGENCE OF MULTIREFERENCE PERTURBATION-METHODS - APPLICATION TO SINGLET-STATES OF CH2

High order perturbative computations for the lowest lying singlet stat es of the CH2 molecule are used to analyze the efficacy of various mul tireference perturbation methods (MRPTs). Whereas traditional Moller-P lesset MRPT calculations produce divergent perturbation expansions, th e effective Hamiltonian H-upsilon and intermediate Hamiltonian H-int a pproaches produce well behaved expansions for well-chosen reference sp aces. The three methods are compared to assess their convergence prope rties, the sources of divergence when appropriate, their accuracy when truncated at low orders, and their behavior when applied in conjuncti on with large reference spaces. The analysis of the sources of diverge nt or slowly convergent perturbation expansions provides insights into necessary ingredients for useful MRPT methods as well as into possibl e approaches for further improving these methods. Calculations are als o presented for a simple problem whose divergent traditional MRPT pert urbation expansion mimics that commonly encountered when these methods are applied in transition state or bond breaking regions of potential surfaces. (C) 1997 American Institute of Physics. [S0021-9606(97)0194 1-7].