Extensive ab initio study of the electronic states of S2 molecule including spin-orbit coupling

The potential energy curves (PECs) of 15 -S states and 24 states generated from the 13 -S bound states of the S-2 molecule are investigated in detail using an ab initio quantum chemical method. The PECs are calculated for internuclear separations from 0.12 to 1.10nm by the complete active space self-consistent field method, which is followed by the internally contracted multireference configuration interaction approach with the Davidson modification (MRCI+Q). The spin-orbit (SO) coupling effect is accounted for by the BreitPauli Hamiltonian. To discuss the effect on the energy splitting by the core-electron correlations, the all-electron basis set, cc-pCVTZ with and without 2s2p correlations, is used for the SO coupling calculations of the A(3) and B-g(3) -S states since their measurements can be found in the literature. By comparison, the cc-pCVTZ basis set with 2s2p correlations is chosen for the SO coupling calculations of 13 -S bound states. To improve the quality of PECs, core-valence correlation and scalar relativistic corrections are included. Scalar relativistic correction calculations are made using the third-order DouglasKroll Hamiltonian (DKH3) approximation at the level of a cc-pV5Z basis set. Core-valence correlation corrections are taken into account with a cc-pCVTZ basis set. The spectroscopic parameters of 13 -S bound states and 24 states are calculated. With the PECs obtained by the MRCI+Q/aug-cc-pV6Z+CV+DK+SO calculations, the SO coupling splitting energies are 379.25cm(1) between the A3 and A2 state, 83.40cm(1) between the A1 and A0 state and 210.91cm(1) between the B2 and B1 state, which agree well with the corresponding measurements of 383, 77.51 and 209cm(1), respectively. Moreover, other spectroscopic parameters are also in excellent agreement with the measurements. It demonstrates that the spectroscopic parameters of 24 states reported here for the first time can be expected to be reliable predicted ones.