Comparison of high order perturbative convergence of multireference perturbation methods: Application to singlet states of CH2

摘要: High order perturbative computations for the lowest lying singlet states of CH2 molecule are used to analyze efficacy various multireference perturbation methods (MRPTs). Whereas traditional Moller–Plesset MRPT calculations produce divergent expansions, effective Hamiltonian Hv and intermediate Hint approaches well behaved expansions well-chosen reference spaces. The three compared assess their convergence properties, sources divergence when appropriate, accuracy truncated at low orders, behavior applied in conjunction with large analysis or slowly convergent provides insights into necessary ingredients useful as possible further improving these methods. Calculations also presented a simple problem whose expansion mimics that commonly encounte...