Ab initio MO studies of neutral and anionic SiC_n clusters (n=2-5)

The geometries and energies of SiC_n and SiC_n (n=2-5) were investigated with ab initia calculations including electron correlation effects with the MP2/6-31G* method, followed by MP4 and CCSD(T) single-point calculations to determine the most stable isomers. The adiabatic electron affinities (AEA) were evaluated with the ΔMP4 method. Because of the complexity of the electronic structure of SiC₂, its AEA was calculated at the CCSD(T)/aug cc-pVTZ//CCSD(T)/aug cc-pVDZ level of approximation. For the neutral SiC_n clusters, the isomer having a triplet ground state has large EA, whereas the isomer having a singlet ground state has small EA. This is attributed to the bonding character between Si and C atoms in the orbital occupied by the extra electron. The calculated EA was used to assign the photoelectron spectra of SiC_n^- reported previously. Furthermore, the effects of Renner-Teller splitting for the linear isomers of anions are discussed.