Applications of perturbation theory to the chemical problems potential energy curves of BH, F2and N2

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Abstract
The many-body Rayleigh-Schrödinger perturbation theory (MB RSPT) truncated at third order in energy is used to calculate potential energy curves and spectroscopic constants of boron hydride and molecular fluorine. It is shown that correlation effects, accounted even at the simplest levels of MB RSPT, bring considerably better agreement of spectroscopic constants with experimental values than that obtained using the pure self-consistent-field (SCF) method. The correlation contributions to the potential energy curves of BH, F2 and N2 are analysed. For N2 the perturbation expansion, up to third order, breaks down for larger internuclear distances. In this case, the variational-perturbation expression or [2/1] Padé approximant is recommended.