Q.-M. Zhang

QUANTUM MOLECULAR DYNAMICS OF CLUSTERS

Recent quantum molecular dynamics studies of Al and carbon clusters are described. For Al, we focused on the 13- and 55-atom clusters, which can assume perfect icosahedral and cubic structures. However, the distortions from these ideal structures are substantial. For the 55-atom cluster, several inequivalent but nearly energetically degenerate structures are found, due to the short range of the screened interatomic interactions. For solid C60, it is found that the soccerball structure is well-preserved in the solid. The intermolecular interactions are so weak that the individual C60 can rotate at relatively low temperatures. At high temperatures vibrations cause large distortions, but the cage structure is still preserved. The C60 isomer containing two pairs of adjacent five-fold rings has a binding energy only 1.6 eV smaller than that of perfect C60, but the transformation between these two structures is hindered by a 5.5 eV barrier. It thus requires high temperatures and long annealing times. High temperatures are also needed for the transformation of the lowest energy C20 isomer, a dodecahedron, to a corannulene structure, which can be thought of as a fragment of C60. The corannulene structure is a natural precursor for the formation of C60. These results are consistent with the experimental findings that high temperatures are necessary for the formation of substantial quantities of C60. A formulation and the first applications of a new, real space quantum molecular dynamics method, particularly suitable for cluster calculations, are also described.

STRUCTURE, DYNAMICS, AND FORMATION OF CARBON AND ALUMINUM CLUSTERS

The results of recent ab initio molecular dynamics studies of C and Al clusters are presented. The simulations have shown that C60 molecular structure is well preserved in the solid and that the individual C60 molecules start to rotate at relatively low temperatures. Our results are in very good agreement with NMR, photoemission, and neutron scattering data. At high temperatures C60 undergoes large amplitude soccerball-rugbyball oscillations, but the cage structure is still preserved. The C60 isomer containing two pairs of adjacent pentagons has a binding energy only 1.6 eV smaller than that of perfect C60, but high temperatures and long annealing times are required for the transformation between these two structures. Its activation energy is 5.4 eV. We have also studied the various isomers of C20, since it could form the smallest possible fullerene. At T=0, the lowest energy isomer is indeed a dodecohedral structure. However, high temperatures favor the corannulene structure, which is a perfect precursor...