Spectroscopic and molecular properties of 14 selected electronic states of Si2 molecule

The potential energy curves (PECs) of the X3Σg−, D3Πu, a1Δg, b1Πu, H′3Σu−, K3Σu−, 13Σu+, 13Πg, 23Σu+, 23Πg, 33Πg, 33Σu+, 23Πu and 23Σg− electronic states of the Si2 molecule are investigated using the complete active space self-consistent field (CASSCF) method followed by the valence internally contracted multireference configuration interaction (MRCI) approach with the correlation-consistent basis sets of Dunning and co-workers. The effects on the PECs by the core-valence correlation and relativistic corrections are included. The way to consider the relativistic correction is to use the third-order Douglas-Kroll Hamiltonian approximation. The core-valence correlation correction is made with the aug-cc-pCV5Z basis set. And the relativistic correction is performed 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 of all these electronic states are extrapolated to the complete basis set limit by the total-energy extrapolation scheme. Using the PECs, the spectroscopic parameters are determined and compared with those reported in the literature. With these PECs determined by the MRCI+Q/CV+DK+56 calculations, the vibrational levels and inertial rotation constants of the first 20 vibrational states are evaluated and compared with the RKR data for these electronic states when the rotational quantum number J equals zero. On the whole, as expected, the most accurate spectroscopic parameters and molecular constants of the Si2 molecule are determined by the MRCI+Q/CV+DK+56 calculations. And the spectroscopic parameters of the 13Σu+, 13Πg, 23Σu+, 23Πg, 33Πg, 33Σu+, 23Πu and 23Σg− electronic states obtained by the MRCI+Q/CV+DK+56 calculations should be good prediction for future laboratory experiment.

► Effect on the PECs by the core-valence correlation and relativistic corrections is included. ► PECs obtained by the MRCI and MRCI+Q are extrapolated to the CBS limit. ► Spectroscopic parameters of 14 electronic states are determined. ► Molecular constants are calculated by the PECs extrapolated. ► Extrapolation obviously improves the quality of spectroscopic parameters.