Effects of geometric and electronic structure on the finite temperature behavior

We report the equilibrium geometries and the electronic structures of Nan clusters in the size range of n =55–62 using density-functional method. An analysis of the evolutionary trends in their ground state geometries reveals that Na58 has a spherical shape which is driven by the closed-shell nature of the electronic structure. This structure shows a significant large network connected by short bonds among the surface atoms as well as between core and surface atoms, which affects its finite-temperature behavior. By employing ab initio density-functional molecular dynamics, we calculate the specific heat of Na58 and Na57. We observe two distinct features in their specific-heat curves as compared to that of Na55: 1 Both clusters show very broad melting transition. 2 The calculated melting temperature of Na58 is 375 K, the highest one studied so far, and that of Na57 is also relatively high 350 K . Thus, when a cluster has a nearly geometric closed-shell structure as well as a nearly electronic closed-shell one, it shows a high melting temperature. Our calculations clearly bring out the size-sensitive nature of the specific-heat curve in sodium clusters.