Melting in Large Sodium Clusters: An Orbital-Free Molecular Dynamics Study

The melting-like transition in sodium clusters Na N, 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 linearly with 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 ∼240 K) is characterized by the high mobility of atoms located on the cluster surface, and the second, homogeneous melting (at ∼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, with surface melting at ∼130 K and homogeneous melting at ∼240 K. Na 55, a complete two-shell icosahedron, melts in a single stage at ∼190 K. Our results on homogeneous melting for Na 142 and Na 92 are in excellent agreement with recent experimental determinations of melting temperatures and latent heats. However, the experimentally observed enhancement of the melting temperature around N = 55 is not reproduced by the calculations.