The melting limit in sodium clusters

Abstract

Thermodynamic properties of the small sodium clusters $\hbox {Na}_6,\, \hbox {Na}_8$ and $\hbox {Na}_{10}$ have been studied by Born–Oppenheimer molecular dynamics (BOMD) simulations. The simulations were performed with auxiliary density functional theory as implemented in the deMon2k code. This approach has already proved accurate for the calculations of thermodynamic properties of larger sodium clusters. The Nosé–Hoover chain thermostat was applied to control the temperature. BOMD simulations were performed in the temperature range from 250 to 1000 K. The obtained trajectories were analyzed using the multiple-histogram method in order to obtain continuous functions for the energies and heat capacities. For the $\hbox {Na}_6$ and $\hbox {Na}_8$ clusters, besides the fragmentation of the clusters at higher temperature, no other characteristic features in the heat capacity curves are found. On the other hand, a small peak at low temperature was found in the $\hbox {Na}_{10}$ heat capacity curve which is characteristic for molecular melting. Our analysis of the $\hbox {Na}_{10}$ melting shows that electronic structure parameters are better suited than geometrical ones to describe the melting process due to the fluctional nature of the clusters. We find that energetical resorting of the occupied cluster orbitals is characteristic for the $\hbox {Na}_{10}$ cluster melting.