The occurrence of carbon clusters in various scientific fields has led to considerable interest in the prediction of the properties of these molecules both experimentally and theoretically. The earliest efforts to compute the electronic structures of linear carbon clusters were undertaken by Pitzer and Clementi. The paper of Raghavachari and Binkley 3 was amongst the earliest works in the dawn of computational chemistry that discussed the structure and frequencies of small carbon clusters (upto 10 carbon atoms) based on HF and CCS(DT) methods. These calculations were improved by Martin et al. who used the CCSD(T) method with correlation-consistent basis sets. On the side of experiments, Gingerich et al. determined the formation enthalpies of small linear carbon clusters using high-temperature Knudsen effusion mass spectroscopy. The measured mole fractions of various carbon clusters in the carbon vapor at temperatures between 2500K and 3000K were used to determine the equilibrium constants, and, ultimately, to back out the formation enthalpies. A 1998 review paper by Van Orden and Saykally 8 surveyed the properties of ground and excited electronic states of carbon clusters, along with the computational methods and experiments that were designed to obtain these values. Since that year, numerous computational studies of the ground states of C n species have been published. The most notable for C n species is by Karton et at. Using state of the art quantum chemistry methods described in the next section, the estimated uncertainties in the formation enthalpies at 0K for C n (n = 2,10) were estimated to be less than ± 0.5 kcal/mol. The importance of small carbon clusters can be seen when looking at their occurrence in the interstellar medium. Owing to this, a recent review of the band systems of C 2 has been published by Schmidt. Study of excited electronic states of larger carbon clusters like C 4 and C 4 H has been rather