Divergence in Moller-Plesset theory: A simple explanation based on a two-state model

The convergence of the Moller-Plesset expansion is examined for Ne, F-, CH2 , and HF and analyzed by means of a simple two-state model. For all systems , increasing diffuseness of the basis introduces highly excited diffuse bac k-door intruder states, resulting in an an alternating, ultimately divergen t expansion. For F-, the divergence begins already at third order; for the remaining systems, it begins later. For CH2, the low-lying doubly excited s tate leads to a monotonic, slowly decreasing series at lower orders; for th e stretched HF molecule, the low-lying doubly excited states lead to a slow ly undulating series at lower orders. Although the divergence of the Moller -Plesset series does not invalidate the use of the second-order expansion, it questions the use of higher-order Moller-Plesset expansions in quantum-c hemical studies. (C) 2000 American Institute of Physics. [S0021-9606(00)301 22-2].