Ground state isoconfigurational mixing in the V2, VNb, and Nb2 molecules

Ground state spin–orbit effects in the molecules V2, VNb, and Nb2 have been investigated using self-consistent field configuration interaction (SCF/CI) calculations based on the intermediate neglect of differential overlap Hamiltonian parameterized for optical spectroscopy (INDO/S). Accurate results for the spin–orbit splittings of the X 3Σ− ground state of each molecule were obtained using a CI treatment designed to isolate essential correlation of the ground state and the isoconfigurational Σ+1 state causing the splittings. Energies computed for the Σ+1 states are compared to energies obtained from simple two-state perturbative models. It is observed that the consideration of only two states is a large source of error. Previous assignment of a low-lying excited state observed for V2 as the Σg+1 state is not supported. Additional results on excited states of VNb have also been obtained. A Σ-3←X Σ-3 transition observed experimentally for VNb is assigned as a σ*←σ promotion, and the presence of a second excited Σ-3 state of VNb that has not been observed is suggested. An analysis of correlation effects in the INDO/S model is given based on a comparison of results obtained with the approximate essential correlation CI calculations and with larger restricted active space full CI calculations.