STATISTICAL PROPERTIES OF THE DENSE HYDROGEN PLASMA - AN AB-INITIO MOLECULAR-DYNAMICS INVESTIGATION

The hydrogen plasma is studied in the very high density (atomic and me tallic) regime by extensive ab initio molecular dynamics simulations. Protons are treated classically, and electrons in the Born-Oppenheimer frame-work, within the local density approximation lu density functio nal theory. Densities and temperatures studied fail within the strong coupling regime of thr: protons. We address the question of the validi ty of linear screening, and we find it yields a reasonably good descri ption up to r(s) approximate to 0.5, but already too crude for r(s) = 1 (with; r(s) = (3/4 pi rho)(1/3) the ion sphere radius). These values an typical of Jovian planets interiors. Finite-size and Brillouin zon e sampling effects In metallic systems are studied and shown to be ver y delicate also in the fluid (liquid metal) phase. We analyze the low- temperature phase diagram and the melting transition, A remarkably fas t decrease of the melting temperature with decreasing density is found , up to a point when it becomes comparable to the Fermi temperature of the protons. The possible vicinity of a triple point bcc-hcp(fcc)-liq uid is discussed in the region of r(s) approximate to 1.1 and T approx imate to 100-200 K. The fluid phase is studied in detail for several t emperatures The structure of the fluid is found robe reminiscent of th e underlying bce (solid) phase. Proton-electron correlations show a we ak temperature dependence, and proton-proton correlations exhibit a we ll-defined first coordination shell, thus characterizing fluid H in th is regime as an atomic liquid. Diffusion coefficients are computed and compared to the values For the one-component plasma. Vibrational dens ities of states (VDOS) show a plasmon renormalization due to electron screening, and the presence of a plasmon-coupled single-particle mode up to very high temperatures. Collective modes are studied through dyn amical structure factors. In close relationship with the VDOS, the sim ulations reveal the remarkable persistence of a weakly clamped high-fr equency ion-acoustic mode, even under conditions of strong electron sc reening. The possibility of using this observation as a diagnostic for the plasma phase transition to the fluid molecular phase at lower den sities is discussed.