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 (X-1 Sigma(+), a(3)Sigma(+), b(3)Pi, d(3)Delta, C-1 Sigma(-), e3 Sigma(-), D-1 Delta and A(1)Pi) 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 (T-e, D-e, R-e, omega(e)x(e), omega Y-e(e), B-e and alpha(e)) 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+QJDK+56 calculations for these electronic states. (C) 2011 Elsevier B.V. All rights reserved.