Effects of system-size and inner-core 2p states on melting of dense sodium at high pressure: ab initio molecular-dynamics simulation

Abstract

We report the effects of system size on melting of dense sodium at about 100 GPa. We have performed systematic ab initio molecular-dynamics (MD) simulations with the number of atoms at most 250 for bcc and 256 for fcc structures based on 3s-valence-electron model and 2p3s-valence-electron model. We have shown that the inner-core 2p states play important roles at extremely high pressures and that the melting temperature estimated by one-phase approach depends on the system size; in particular, the melting temperature for 3s model decreases with increasing system size and deviates from the experimental value, while the melting temperature for the 2p3s model approaches to the experimental value slowly. This result suggests that we have to carry out large-scale ab initio MD simulations with the inner-core 2p states to obtain the melting temperature of dense Na at extremely high pressure.