Formally Exact Many-Body Perturbation Theory with Optimized Zeroth Approximation in Calculations of Spectral Parameters of Diatomic Molecules

This chapter reviews the fundamentals of effective approach to computing different molecular characteristics (the potential energy curves, molecular constants, dipole moments, probabilities of radiative transitions etc.) of diatomic moleculesDiatomic molecules. The approach is based on new version of a formally exact Rayleigh-Schrödinger many-body perturbation theoryMany-body perturbation theory (PT) with ab initio gauge-invariant model potential zeroth approximation and with effective accounting for the complex many-body exchange–correlation effectsExchange-correlation effects as effects of the PT second and higher orders (including polarization interaction of external quasiparticles through a polarized core, or their mutual screening, iterations of the mass operator, pressure continuum, etc.). In contrast to the known similar PT versions, the fundamental ab initio procedure for construction of a model potential (MP) zeroth approximation is developed. The problem of the construction of the optimal one-electron representation in the theory of the multielectron systems is reduced to the minimization of the gauge dependent multielectron contribution to the radiation widths of electron states (transition probabilities). The perturbation theory first order correction is calculated exactly, the high order contributions are accounted for effectively. Two main types of high order corrections are investigated: “polarization” interaction of two “above core” particles and effect of the mutual screening of these particles. Some illustrative results of calculation of the energy and spectral parametersElectron structure and spectral parameters of some diatomic moleculesDiatomic molecules (the sodium, rubidium and caesium dimers) are presented. The computational results for molecular constants of the investigated molecules are in physically reasonable agreement with both the available experimental data and the results of other calculations using alternative, in particular empirical approaches.