Fourth-order MB-RSPT calculations of the spectroscopic constants and potential energy curve of F2

The spectroscopic constants and the potential energy curve of F₂ were calculated, using the fourth-order MB-RSPT with a single-determinant RHF starting wave function. With an extended [5s4p2d1f] basis set we obtained the equilibrium bond distance and the harmonic vibrational frequency with a relative error of about 0.5%, these are in very good agreement with experiment. In calculations of the potential energy curve for distances larger than about 1.4 R_e the method breaks down. We analysed the effect of the individual fourth-order contributions: single, double, triple and quadruple excitations. The role of the renormalization term was stressed in the discussion of various approximations to the full fourth-order energy and in comparison with other related approaches. The basis set effect has been also examined.