Spin–orbit coupling effects in the ground state and four low-lying excited valence states a1Δ, B3Σ−, 13Δ and 13Σ+ of SeS molecule

The potential energy curves (PECs) of 10 Ω states generated from the five Λ–S valence states of SeS molecule are investigated in detail using an ab initio quantum chemical method for the first time. All the PEC calculations are made by the complete active space self-consistent field method, which is followed by the internally contracted multireference configuration interaction (MRCI) approach with the Davidson modification (MRCI + Q). The spin–orbit coupling effect is included in the present study by the Breit–Pauli Hamiltonian with the aug-cc-pV5Z basis set. Convergent behavior is discussed and excellent convergence is observed with respect to the basis set and level of theory. To improve the quality of PECs, core-valence correlation and scalar relativistic corrections are included. Core-valence correlation corrections are taken into account with a cc-pCVTZ basis set. Scalar relativistic corrections are calculated by the third-order Douglas–Kroll Hamiltonian approximation at the level of a cc-pVQZ basis set. The PECs of all the Λ–S and Ω states are extrapolated to the complete basis set limit by the total-energy extrapolation scheme so that the residual errors behind the basis sets can be minimized. With the PECs including all the corrections used here, the spectroscopic parameters of five Λ–S and 10 Ω states are calculated, which agree well with the measurements. The spectroscopic parameters of five Ω states (13Δ3, 13Δ2, 13Δ1, 1 3 Σ 0 + + and 1 3 Σ 1 + ) reported here for the first time can be expected to be reliable predicted ones.