The melting-like transition in sodium clusters NaN, with N = 55, 92, and 142, is studied by using constant-energy molecular dynamics simulations. An orbital-free version of the Car-Parrinello technique is used that scales linearlywith system size, allowing for investigations of the thermal behavior of large clusters.The ground-state isomer of Na_(142) (an incomplete three-shell icosahedron) melts in two steps: the first (at approx 240K) is characterized by the high mobility of atoms located on the cluster surface, and the second,melting (at approx 270 K) involves diffusive motion of all of the atoms across the cluster. For the case of Na_(92), the icosahedral structure has a larger number of surface vacancies and melts in two well-separated steps, wil surface melting at approx 130 K and homogeneous melting at approx 240 K. Na55, a complete two-shell icosahedra. melts in a single stage at approx 190 K. Our results on homogeneous melting for Na1.and Na_(92) are in excell~ agreement with recent experimental determinations of melting temperatures and latent heats. However, tlu experimentally observed enhancement of the melting temperature around N_(55) is not reproduced by tiu calculations.
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