Structural and vibrational analysis of amorphousAu55clusters

We present a structural and vibrational analysis of several amorphous (disordered) and ordered isomers of a 55-atom gold cluster. A Gupta n-body potential, with parameters fitted to gold clusters, was used to model the metallic bonding in the ${\mathrm{Au}}_{55}$ cluster. The molecular-dynamics method combined with simulated annealing and quenching techniques were used to perform the cluster structure optimization. Our results show that several nonequivalent and nearly degenerate in energy amorphous cluster structures are more stable than those with high symmetry like the 55-atom Mackay icosahedron and the fcc cuboctahedron. The calculated distribution of normal frequencies clearly discriminates between amorphous and ordered cluster configurations and confirms their stability. A common-neighbor analysis was implemented to characterize the disordered cluster structures, identifying the short-range order of the amorphous phase, according to the local environment of each atom pair in the cluster. Distorted multilayer icosahedral order was found to be the more representative of the amorphous clusters with the lowest energies. At higher energies, the amorphous structures are characterized by the presence of distorted local icosahedral order. The origin of the higher stability of amorphous vs ordered isomers in ${\mathrm{Au}}_{55}$ is in the short range of the n-body interaction existing in the metal cluster bonding. \textcopyright{} 1996 The American Physical Society.