STRUCTURES OF SMALL METAL-CLUSTERS .1. LOW-TEMPERATURE BEHAVIOR

The binding energy, surface energy, surface atom coordination numbers, and bond lengths and angles of small clusters at low temperatures are calculated with parameters fitted to Ni using both the Lennard-Jones (LJ) potential and the embedded atom (EA) potential to assess the sensitivity of interatomic potential on cluster structure. Simulations are performed by implementation of the simulated annealing method in a canonical ensemble Monte Carlo technique. We examine clusters with n less-than-or-equal-to 34 atoms and we find that they are noncrystalline (with the exception of n = 6). The most stable structure of clusters consisting of n less-than-or-equal-to 15 atoms and n = 19 atoms is the same for both potentials (with the exception of n = 8). However, the most stable structure of clusters with n greater-than-or-equal-to 16 atoms is different for the two potentials (with the exception of n = 19). Smeared angular distribution and pair distribution functions are found for many EA clusters whereas sharp, well defined peaks exist for LJ clusters. A discontinuous transition from polyicosahedral to quasicrystalline structure is found from n = 30 to n = 31 atoms for the LJ potential. This transition occurs at smaller n for the EA potential. Surface atom coordination numbers are found to be nonmonotonic functions of cluster size. The existence of multiple structures of small clusters and the effect of quenching rate during crystallization on the final shape of clusters are also examined.