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

Abstract

Full configuration‐interaction (FCI) calculations have been performed for the X ¹Σ⁺_g, a ³Π_u, and b ³Σ⁻_g states of C₂ and the X ³Σ⁻_g and A ³Π_u 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 ³Π_u 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 r_e=1.719 Å, ω_e=962 cm⁻¹ and D_e=4.4 eV.