A b i n i t i o calculations on C2, Si2, and SiC

Full configuration-interaction (FCI) calculations have been performed for the X 1Σ+g, a 3Πu, and b 3Σ−g states of C2 and the X 3Σ−g and A 3Πu states of Si2 in valence double-zeta plus polarization one-particle basis sets. Of the various approximate correlation treatments compared to the FCI, the CASSCF/MRCI method is found to be far superior in reproducing the FCI separations between the low-lying states. Hence this procedure was taken to near the one-particle basis set limit using 5s4p3d2f1g carbon and 6s5p3d2f1g silicon atom Gaussian basis sets contracted using the newly devised atomic natural orbital (ANO) method. The computed C2 separations in the large basis set are in excellent agreement with experiment. The ground state of Si2 is definitively determined to be X 3Σ−g, but the A 3Πu state is only 440±100 cm−1 higher in energy. For SiC, the A 3Σ− state is nearly 4000 cm−1 above the X 3Π ground state. Our best estimates of the spectroscopic parameters for the X 3Π state of SiC are re=1.719 Å, ωe=962 cm−1 and De=4.4 eV.