The geometries and energies of SiCn and SiCn- (n=2-5) were investigated with ab initio 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 Delta MP4 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 SiCn 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 SiCn- reported previously. Furthermore, the effects of Renner-Teller splitting for the linear isomers of anions are discussed. (C) 1997 American Institute of Physics. [S0021-9606(97)02247-2].