Theoretical study of the isovalent diatomics C2, Si2, and SiC

Ab initio calculations on the molecules C2, Si2, and SiC are performed to determine equilibrium geometries (R/sub e/) and spectroscopic constants (omega/sub e/, omega/sub e/chi/sub e/, and D/sub e/) for various electronic states. A large basis set including two d functions on each atom is used and electron correlation is treated by two different methods. The first approach is that of Moeller-Plesset perturbation theory based on a UHF reference function. The second approach is that of externally contracted configuration interaction based on a multireference function of the complete-active-space type. Good agreement between theory and experiment is achieved for the homonuclear diatomics. The best theoretical value obtained for the ground-state harmonic frequency omega/sub e/ of SiC is 940 cm-1. After consideration of the remaining basis set and correlation effects, an estimate of 975 +/- 10 cm-1 is made for omega/sub e/ in the unobserved molecule SiC. Finally, a comparison is made between the two theoretical approaches used in this study. 22 references, 3 tables