Wave-Packet Molecular Dynamics Simulation of Hydrogen

Abstract

Hydrogen, being composed of the simplest atom, is a material whose properties have attracted attention since long. It was as early as 1935 that the existence of a metallic phase at high pressures has been postulated. Since then this problem has been intensely studied, not only as a basic challenge in many-body physics but also because of important astrophysical implications. Recent experiments with multiple shock waves show a remarked increase of the conductivity by four orders of magnitude for pressures between 93 and 180 In these experiments neither the density nor the temperature have been measured independently. Instead they were calculated by computationally simulating each measurement using a standard equation of state for hydrogen in the molecular fluid phase. Relating the resistivity to a density-dependent band gap and to the temperature, the data have thus been interpreted in terms of a continuous transition from a semiconducting to a metallic diatomic fluid at 140 GPa and 3000 K. However, this view does not comply with several theoretical investigations using different advanced many-body methods which all predict a first-order phase transition. A chemical picture has been employed in [5] where hydrogen is described as a mixture of H2, H, H + and free electrons where the properties of molecules, atoms and ions change with increasing density because of the polarization due to strong coupling. Hierarchical schemes based on the hypernetted chain (HNC) equations allow to account for both longand short-range correlations in fluids and these have been applied to the hydrogen problem. Using the equation of state emerging therefrom and corresponding models for the conductivity, the results of the multiple shock wave experiments have been interpreted as a first-order transition between molecular and metallic phases. This view is supported by path-integral Monte Carlo (PIMC) studies of hydrogen which show a decreasing pressure P(T) as the temperature is raised between 5000 K and 10000 K at densities around This drop in pressure is due to the decrease in electron kinetic energy density upon the breaking of the molecular bonds during the dissociation of the H2 molecules. A tight binding calculation shows no such drop in pressure, on the other hand. Here we investigate the problem of a phase transition in hydrogen at high pressures on the basis of extensive numerical simulations of the equation of state, pair correlation functions and charge transport coefficients. To this end, we employ an efficient quantum mechanical simulation scheme, the wave-packet molecular dynamics (WPMD). The technical simplifications of the scheme allow to use much larger samples which, in turn, help to pin down