USE OF MOLLER-PLESSET PERTURBATION-THEORY IN MOLECULAR CALCULATIONS -SPECTROSCOPIC CONSTANTS OF FIRST ROW DIATOMIC-MOLECULES

The convergence of Moller-Plesset perturbation expansions (MP2-MP4/MP5 ) for the spectroscopic constants of a selected set of diatomic molecu les (BH, CH, HF, N-2, CO, and F-2) has been investigated. It was found that the second-order perturbation contributions to the spectroscopic constants are strongly dependent on basis set, more so for HF and CO than for BH. The MP5 contributions for HF were essentially zero for th e cc-pVDZ basis set, but increased significantly with basis set illust rating the difficulty of using small basis sets as benchmarks for corr elated calculations. The convergence behavior of the exact Moller-Ples set perturbation expansions were investigated using estimates of the c omplete basis set limits obtained using large correlation consistent b asis sets. For BH and CH, the perturbation expansions of the spectrosc opic constants converge monotonically toward the experimental values, while for HF, N-2, CO, and F-2, the expansions oscillate about the exp erimental values. The perturbation expansions are, in general, only sl owly converging and, for HF, N-2, CO, and F-2, appear to be far from c onvergence at MP4. In fact, for HF, N-2, and CO, the errors in the cal culated spectroscopic constants for the MP4 method are larger than tho se for the MP2 method (the only exception is D-e). The current study, combined with other recent studies, raises serious doubts about the us e of Moller-Plesset perturbation theory to describe electron correlati on effects in atomic and molecular calculations. (C) 1998 American Ins titute of Physics.