COPPER CLUSTERS SIMULATED BY A MANY-BODY TIGHT-BINDING POTENTIAL

We have estimated the classical ground states of copper clusters consisting of 13-1289 atoms using a tight-binding many-body potential. Starting from perfect face-centred-cubic (f.c.c.) and icosahedral structures, each system was allowed to evolve toward its inherent structure (that is the nearest local minimum energy configuration at T=OK). The (relaxed) icosahedra turned out to be more cohesive for all the directly simulated systems. We have also estimated the critical size at which the fc.c. structure becomes energetically favourable performing nuclear fits of the ground-state energy for both structures. The icosahedral configuration is more stable for clusters containing less than about 1500 atoms whereas fc.c. structures are preferred for clusters of larger size. From the simulated phonon spectra, one has that the mean square displacement of an atom depends on the shell it belongs to and that, at least in the pure harmonic approximation, atoms near the centre of the cluster perform small oscillations than those near the cluster boundaries.