Density functional study of carbon clusters C-2n (2 <= n <= 16). I. Structure and bonding in the neutral clusters

Density functional calculations have been performed for many isomers of neu tral carbon clusters C-n (4 less than or equal to n less than or equal to 3 2, n even) using both local spin density (LSD) and gradient-corrected (Beck e-Perdew) approximations to the exchange-correlation energy. The stable iso mers include chains, rings, cages, and graphitic ("plate'' and "bowl'') str uctures, and we observe a fourfold periodicity in several structural classe s as n changes. Stable cages exist for all clusters with n greater than or equal to 8, and double rings are less stable than the monocyclic rings in a ll cases. Most planar structures have low-frequency out-of-plane vibrations . Gradient corrections often change the ordering of the energies of the iso mers, but the effects are remarkably regular within a given structural type . (C) 1999 American Institute of Physics. [S0021-9606(99)30210-5].