Ab initio MRCI + Q study on potential energy curves and spectroscopic parameters of low-lying electronic states of CS⁺

Carbon monosulfide molecular ion (CS⁺), which plays an important role in various research fields, has long been attracting much interest. Because of the unstable and transient nature of CS⁺, its electronic states have not been well investigated. In this paper, the electronic states of CS⁺ are studied by employing the internally contracted multireference configuration interaction method, and taking into account relativistic effects (scalar plus spin—orbit coupling). The spin—orbit coupling effects are considered via the state-interacting method with the full Breit—Pauli Hamiltonian. The potential energy curves of 18 Λ—S states correlated with the two lowest dissociation limits of CS⁺ molecular ion are calculated, and those of 10 lowest Ω states generated from the 6 lowest Λ—S states are also worked out. The spectroscopic constants of the bound states are evaluated, and they are in good agreement with available experimental results and theoretical values. With the aid of analysis of Λ—S composition of Ω states at different bond lengths, the avoided crossing phenomena in the electronic states of CS⁺ are illuminated. Finally, the single ionization spectra of CS (X¹Σ⁺) populating the CS⁺(X²Σ⁺_1/2, A²Π_3/2, A²Π_1/2, and B²Σ⁺_1/2) states are simulated. The vertical ionization potentials for X²Σ⁺_1/2, A²Π_3/2, A²Π_1/2, and B²Σ⁺_1/2 states are calculated to be 11.257, 12.787, 12.827, and 15.860 eV, respectively, which are accurate compared with previous experimental results, within an error margin of 0.08 eV∼0.2 eV.