Ab initio Gibbs ensemble Monte Carlo simulations of the liquid-vapor equilibrium and the critical point of sodium.

The ab initio (ai) Gibbs ensemble (GE) Monte Carlo (MC) method coupled with Kohn-Sham density functional theory is successful in predicting the liquid-vapour equilibrium of insulating systems. Here we show that the aiGEMC method can be used to study also metallic systems, where the excited electronic states play an important role and cannot be neglected. For this we include the electronic free energy in the formulation of the effective energy of the system to be used in the acceptance criteria for the MC moves. The application of this aiGEMC method to sodium yields a good agreement with available experimental data on the liquid-vapour equilibrium densities. We predict a critical point for sodium at 2338 ± 108 K and 0.24 ± 0.03 g cm-3. The liquid structure stemming from aiGEMC simulations is very similar to the one from ab initio molecular dynamics. Since this method can determine phase transition without computing the Gibbs free energy, it may offer a new possibility to study other materials with a reasonable computational cost.

Free-energy model for fluid helium at high density.

We present a semianalytical free-energy model aimed at characterizing the thermodynamic properties of dense fluid helium, from the low-density atomic phase to the high-density fully ionized regime. The model is based on a free-energy minimization method and includes various different contributions representative of the correlations between atomic and ionic species and electrons. This model allows the computation of the thermodynamic properties of dense helium over an extended range of density and temperature and leads to the computation of the phase diagram of dense fluid helium, with its various temperature and pressure ionization contours. One of the predictions of the model is that pressure ionization occurs abruptly at rho greater, > or = 10 g cm(-3) , i.e., P greater, > or = 20 Mbar , from atomic helium He to fully ionized helium He2+ , or at least to a strongly ionized state, without a He+ stage, except at high enough temperature for temperature ionization to become dominant. These predictions and this phase diagram provide a guide for future dynamical experiments or numerical first-principle calculations aimed at studying the properties of helium at very high density, in particular its metallization. Indeed, the characterization of the helium phase diagram bears important consequences for the thermodynamic, magnetic, and transport properties of cool and dense astrophysical objects, among which are the solar and the numerous recently discovered extrasolar giant planets.

Microcanonical calculations of excess thermodynamic properties of dense binary systems.

We derive a formulation to calculate the excess chemical potential of a fraction of N1 particles interacting with N2 particles of a different species. The excess chemical potential is calculated numerically from first principles by coupling molecular dynamics and Thomas-Fermi density functional theory to take into account the contribution arising from the quantum electrons on the forces acting on the ions. The choice of this simple functional is motivated by the fact that the present paper is devoted to the derivation and the validation of the method but more complicated functionals can and will be implemented in the future. This method is applied in the microcanonical ensemble, the most natural ensemble for molecular dynamics simulations. This avoids the introduction of a thermostat in the simulation and thus uncontrolled modifications of the trajectories calculated from the forces between particles. The calculations are conducted for three values of the input thermodynamic quantities, energy and density, and for different total numbers of particles in order to examine the uncertainties due to finite-size effects. This method and these calculations lie the basic foundation to study the thermodynamic stability of dense mixtures, without any a priori assumption on the degree of ionization of the different species.