Accurate spectroscopic constants of the lowest two electronic states in S_2 molecule with explicitly correlated method

A computational scheme for accurate spectroscopic constants was presented in this work and applied to the lowest two electronic states of sulfur dimer. A high-level ab initio calculation utilizing explicitly correlated multireference configuration interaction method (MRCI-F12) was performed to compute the potential energy curves (PECs) of the ground triplet X³Σ_g^- and first excited singlet a¹Δ^g states of sulfur dimer with cc-pCVXZ-F12(X=m T, Q) basis sets. The effects of Davidson modification, core-valence correlation correction, and scalar relativistic correction on the spectroscopic constants were examined. The vibration-rotation spectra of the two electronic states were provided. Our computational results show excellent agreement with existing available experimental values, and the errors of main spectroscopic constants are within 0.1% order of magnitude. The present computational scheme is cheap and accurate, which is expected for extensive investigations on the potential energy curves or surfaces of other molecular systems.