Study of the H2/H-He mixtures at extreme conditions: demixing, insulator-metal transition and miscibility boundaries*

Accurate knowledge of the H-He demixing conditions and location of the insulator-metal-transition (IMT) boundary at Mbar pressures is important for planetary models. The most successful approach, to date, is the evaluation of the Gibbs free-energy of mixing based on the non-ideal entropy of mixing. Here, we explore a different approach. Using an NPT ensemble, large-scale ab-initio molecular dynamics (AIMD) simulations are performed along a set of selected isobars. At low-T the system spontaneously separates resulting in two regions, one that is He-rich and one that is molecular hydrogen-rich. Calculations of the conductivity indicate this demixed system is an insulator. Admixture with a small fraction of He makes the H2 bonds stronger leading to a H2 dissociation and related IMT that occurs at temperatures significantly higher as compared to pure H2. With further increase of temperature the H-He mixing will occur. Analysis of the first peak of the H-He radial distribution function provides a sharp signature of the transition from the demixed to the perfectly mixed phase. The use of a thermal exchange-correlation (XC) functional in our AIMD simulations shifts the miscibility boundary by about 10% up in temperature as compared to the standard ground-state XC functional.