Hydrogen and hydrogen-helium mixtures under high pressure: a density functional and molecular dynamics study

What kind of element is hydrogen? Its deceptively simple composition of electrons and protons contrasts with a miscellany of displayed phenomena and a protean versatility. Its special role among the elements continues to fascinate a large community of physicists. On one hand hydrogen is only one electron short of having a closed shell and ranks with fluorine, chlorine and the halogens as group VII elements. Similar to those elements it forms tightly bound molecules under normal conditions, the bond being mediated by the sharing of electrons (covalency). On the other hand one can regard hydrogen as a group I element. Like the alkali metals it has one (outer) electron at its disposal for chemical interactions. However it is a "reluctant alkali" [9] and the analogy only holds at high pressures. In the Mbar pressure range the H moleeules give up their identity and the pressure transforms molecular, insulating hydrogen into an atomic metal. Since the required pressures are at the limit of or beyond current experimental capabilities ($\sim$ 2.5 Mbar), much of our knowledge about the high pressure phases of hydrogen relies on theoretical investigations. It has been pointed out that the complex behavior of hydrogen in the molecular-atomic transition region defies treatment with simple, empirical models. Instead, accurate simulations require the full quantum mechanical determination of the interaction, e.g. in the framework of density functional theory [12]. Here we apply a combination of molecular dynamics and density functional theory (Car-Parrinello method) to study the high pressure behavior of hydrogen. This method allows us to investigate hydrogen in a wide pressure range ($\sim$ 1 - 100 Mbar). We examine the gradual dissociation of the hydrogen molecules, the change in the ground state structure, and the on set of metallization. At the highest pressure we enter the domain of plasma physics. In the universe the high pressure phases of hydrogen are realized e.g. in the giant planets, Jupiter and Saturn, which consist mainly of hydrogen and a smaller amount of helium. Presently Jupiter is in the focus of attention through "the Galileo Mission" [89], i.e. the investigation of the planetary atmosphere by the Galileo spacecraft. For the interpretation of the collected data, knowledge of the physics of the interior in these planets is indispensable. For example, the large magnetosphere of Jupiter is due to the metallization of hydrogen in the inner envelope of Jupiter. Its high pressure properties, e.g. the conductivities of metallic [...