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The thermodynamic behavior of near ground-state isomers of Au55 clusters are investigated through molecular-dynamic simulations using an n-body effective potential. During relaxation at certain energies, non-reversible structural transitions (from cuboctahedron to icosahedron, and then to disordered configuration) occur well below the melting temperature. We show in this Letter on a picosecond scale the motion of the individual atoms in a cluster during these transformations, demonstrating that the processes involved are either collective distortions of the entire cluster or migration of individual or small groups of atoms. In both cases the cohesive energies of the system go smoothly without any significant barrier during transitions. [Copyright &y& Elsevier]