The melting-like transition in sodium clusters Na_N, with N=55, 92, and 142is studied by using constant-energy molecular dynamics simulations. Anorbital-free version of the Car-Parrinello technique is used which scaleslinearly with system size allowing investigation of the thermal behaviour oflarge clusters. The ground state isomer of Na_142 (an uncomplete three-shellicosahedron) melts in two steps: the first one (at approx. 240 K) ischaracterized by the high mobility of the atoms located on the cluster surface;the second, homogeneous melting (at approx. 270 K), involves diffusive motionof all the atoms across the cluster. For the case of Na_92, the icosahedralstructure has a larger number of surface vacancies, and melts in two wellseparated steps, surface melting at approx. 130 K and homogeneous melting atapprox. 240 K. Na_55, a complete two-shell icosahedron, melts in a single stageat approx. 190 K. Our results on homogeneous melting for Na_142 and Na_92 arein excellent agreement with recent experimental determinations of meltingtemperatures and latent heats. However, the experimentally observed enhancementof the melting temperature around N=55 is not reproduced by the calculations.
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