Electronic and molecular structures of medium-sized carbon clusters

Very accurate ab initio calculations with large basis sets were carried out to determine electronic and molecular structures of C{dollar}sb4{dollar}, C{dollar}sb6{dollar}, C{dollar}sb8{dollar}, C{dollar}sb{lcub}10{rcub}{dollar} and C{dollar}sb{lcub}18{rcub}{dollar}. For the smaller clusters, i.e. C{dollar}sb4{dollar}, C{dollar}sb6{dollar} and C{dollar}sb8{dollar}, linear {dollar}sp3Sigmasb{lcub}rm g{rcub}sp{lcub}-{rcub}{dollar} state of cumulene) and monocyclic structures are nearly isoenergetic. For the larger clusters, i.e C{dollar}sb{lcub}10{rcub}{dollar} and C{dollar}sb{lcub}18{rcub}{dollar}, cyclic structures are the ground-state conformers with energies much lower than the linear forms. Basis sets and methods used appear to be very important for determining energy differences between different conformers of carbon clusters.