Theoretical study on thirteen Λ-S states of the SiN radical: potential energy curves, spectroscopic parameters and spin-orbit couplings

The spectroscopic properties of nineteen Omega states generated from the eight Lambda-S bound states of SiN radicals are studied in detail using an ab initio quantum chemical method for the first time. The potential energy curves (PECs) are calculated by the complete active space self-consistent field method, which is followed by the internally contracted multireference configuration interaction approach with the Davidson modification (icMRCI+Q). The spin-orbit coupling is accounted for by the Breit-Pauli Hamiltonian using the cc-pCVTZ basis set. Scalar relativistic correction calculations are made using the third-order Douglas-Kroll Hamiltonian approximation at the level of a cc-pV5Z basis set. Core-valence correlation corrections are included with a cc-pCVTZ basis set. All the PECs are extrapolated to the complete basis set limit. The convergent behavior is discussed with respect to the basis set and level of theory. The effect of core-electron correlations on the energy splitting is studied. Using the PECs, the spectroscopic parameters of twelve Lambda-S and nineteen Omega bound states are evaluated. At the icMRCI+Q/CV+DK+56 theoretical level, the spin-orbit coupling splitting energy of the A (2)Pi Lambda-S state is found to be 88.45 cm(-1), which agrees well with the recent measurements of 89.09 cm(-1). Moreover, other spectroscopic parameters are also in fair agreement with their respective measurements. The effect of SO coupling on the vibrational levels is discussed. It shows that the spectroscopic parameters of the twelve Lambda-S and nineteen Omega bound states reported here can be expected to be reliably predicted ones.