Model dependence of the thermodynamic properties of Na<SUB>8</SUB> and Na<SUB>20</SUB> clusters studied with ab initio electronic structure methods

We study the thermodynamics of Na8 and Na20 clusters using multiple-histogram methods and an ab initio treatment of the valence electrons within density functional theory. We consider the influence of various electron kinetic-energy functionals and pseudopotentials on the canonical ionic specific heats. The results for all models we consider show qualitative similarities, but also significant temperature shifts from model to model of peaks and other features in the specific-heat curves. The use of phenomenological pseudopotentials shifts the melting peak substantially (~50-100 K) when compared to ab initioresults. It is argued that the choice of a good pseudopotential and use of better electronic kinetic-energy functionals has the potential for performing large-time-scale and large-size thermodynamical simulations on clusters