Ab initio calculations, typically at the MP2/DZP level, have in several cases shown that cage structures of the fullerene type are energetically favored over cyclic geometries. As this appears to be at variance with experimental findings, it is of interest to investigate whether the MP2/DZP procedure overestimates the stability of fullerenes. For the C and BN model systems, calculations at two geometries (a cubane-like cluster and a monocyclic ring) with basis sets up to 6-311 G (3d2f) and electron correlation up to CCSD (T) show that the MP2/DZP procedure gives results in reasonable agreement with those obtained at more sophisticated levels. It is suggested that the experimentally observed dominance of cyclic isomers is due to entropy contributions to the free energy.
