Ab Initio MRCI+Q Study on Low-Lying States of CS Including Spin–Orbit Coupling

Carbon monosulfide (CS), which plays an important role in a variety of research fields, has long received considerable interest. Due to its transient nature and large state density, the electronic states of CS have not been well understood, especially the interactions between different states. In this paper, we performed a detail ab initio study on the low-lying electronic states of CS by means of the internally contracted multireference configuration interaction method (including Davidson correction) with scalar relativistic correction using the Douglas–Kroll–Hess Hamiltonian. We focused on the spin–orbit coupling of the states via the state interaction method with the full Breit–Pauli Hamiltonian. The potential energy curves (PECs) of 18 Λ–S states correlated with the lowest dissociation limit of the CS molecule were calculated, as well as those of 50 Ω states generated from the Λ–S states. The spectroscopic constants of the bound states were obtained, which are in good agreement with previous available experimental and theoretical results. The state perturbations of the a3Π and A1Π states with other low-lying electronic states are discussed in detail, based on the calculated spin–orbit matrix as well as the PECs of the Ω states. Avoided crossing in the states of CS was indicated when spin–orbit coupling was taken into account. Finally, the allowed transition dipole moments as well as the lifetimes of the five lowest vibrational states of the A1Π1, A′1Σ+0+ and a3Πi states were obtained.