Spectroscopic and molecular properties of X1Σ+, a3Σ+, b3Π, d3Δ, C1Σ−, e3Σ−, D1Δ and A1Π electronic states of SiS molecule

The potential energy curves (PECs) of eight low-lying electronic states (X1Σ+, a3Σ+, b3Π, d3Δ, C1Σ−, e3Σ−, D1Δ and A1Π) of SiS molecule were investigated with the full valence complete active space self-consistent field (CASSCF) method followed by the highly accurate valence internally contracted multireference configuration interaction (MRCI) approach with the correlation-consistent basis sets, aug-cc-pV5Z and aug-cc-pV6Z. Effect on the PECs by the relativistic correction is taken into account. The way to consider the relativistic correction is to use the second-order Douglas–Kroll Hamiltonian (DKH2) approximation. And the relativistic correction is made at the level of cc-pV5Z basis set. To obtain more reliable results, the PECs determined by the MRCI calculations are also corrected for size-extensivity errors by means of the Davidson modification (MRCI+Q). The PECs are extrapolated to the complete basis set (CBS) limit by the two-point total-energy extrapolation scheme. With these PECs, the spectroscopic parameters (Te, De, Re, ωe, ωexe, ωeуe, Be and αe) are determined and compared with those reported in the literature. With the PECs obtained by the MRCI+Q/DK+56 calculations, the vibrational levels and inertial rotation and centrifugal distortion constants of the first 30 vibrational states are calculated for each electronic state involved here when the rotational quantum number J is equal to zero. Comparison with the measurements shows that the present total-energy extrapolation could improve the quality of spectroscopic parameters. On the whole, as expected, the spectroscopic parameters and molecular constants closest to the experimental data are determined by the MRCI+Q/DK+56 calculations for these electronic states.