The Shape of Bucky Onions

The morphology of Bucky onions is investigated by {\it ab initio} calculations using Yang's new $O(N)$ method. It is found that for large single shell fullerenes with $I_h$ symmetry, the spherical morphology has the lower energy than that of polyhedrons. The formation energy per atom follows a simple scaling law. Including an estimate of inter-shell van de Waals interactions leads to the conclusion that spherical multiple shell Bucky onions are likely the most stable structure of large carbon clusters. These results are in good agreement with recent observation.