Low-lying [formula omitted] electronic states of BiF: Perturbative versus variational approaches of spin–orbit coupling.

The low-lying Ω states of bismuth monofluoride (BiF) below 50000 cm−1 have been theoretically studied using the multi-reference configuration interaction and the equation-of-motion coupled-cluster methods, where the spin–orbit coupling (SOC) effects are respectively considered perturbatively or variationally. Since the perturbative treatment of SOC is not a good approximation for the heavy atom Bi, the latter method exhibits higher accuracy and better agreements with the experimental spectroscopic constants. Our results show that the second 3 Π state of BiF is due to the occupation on the Rydberg 7 s -shell of Bi, which gives rise to the so-called "triplet C " state and its sub-states reported in the early literatures. With the help of our theoretical results, some experimentally assigned spectral bands with considerable confusion have also been clarified and reassigned. [Display omitted] • The SOC effects have been included through perturbative and variational ways. • All the valence and Redberg Ω states below 50000 cm-1 have been investigated. • Some uncertain spectral transitions in the literatures have been reassigned. • The dissociation energy is calculated to be 4.09 eV at the complete basis set limit. [ABSTRACT FROM AUTHOR]