It is generally assumed 1,2,3 that solid hydrogen will transform into a metallic alkali-like crystal at sufficiently high pressure. However, some theoretical models 4,5 have also suggested that compressed hydrogen may form an unusual two-component (protons and electrons) metallic fluid at low temperature, or possibly even a zero-temperature liquid ground state. The existence of these new states of matter is conditional on the presence of a maximum in the melting temperature versus pressure curve (the 'melt line'). Previous measurements 6,7,8 of the hydrogen melt line up to pressures of 44 GPa have led to controversial conclusions regarding the existence of this maximum. Here we report ab initio calculations that establish the melt line up to 200 GPa. We predict that subtle changes in the intermolecular interactions lead to a decline of the melt line above 90 GPa. The implication is that as solid molecular hydrogen is compressed, it transforms into a low-temperature quantum fluid before becoming a monatomic crystal. The emerging low-temperature phase diagram of hydrogen and its isotopes bears analogies with the familiar phases of 3 He and 4 He, the only known zero-temperature liquids, but the long-range Coulombic interactions and the large component mass ratio present in hydrogen would ensure dramatically different properties 9,10 .The possible existence of low-temperature liquid phases of compressed hydrogen has been rationalized with arguments based on the nature of effective pair interactions and of the quantum dynamics at high density, resulting in proton-proton correlations insufficient for the stabilization of a crystalline phase 5 . But so far there has been no conclusive evidence establishing whether hydrogen metallizes at low temperature as a solid (the more widely accepted view to date) or as a liquid. Measurements and theoretical predictions of the near-ground state high-pressure phases 3 of hydrogen have proven to be difficult because of the light atomic mass, significant quantum effects and strong electron-ion interactions. In this regard, the finite temperature liquid-solid phase boundary predicted here is especially valuable for understanding the manner in which hydrogen metallizes.The appearance of a maximum melting temperature in hydrogen is in itself a manifestation of an unusual physical phenomenon. The few systems with a negative melt slope involve either open crystalline structures, such as water and graphite, or in the case of closed packed solids, a promotion of valence electrons to higher orbitals upon compression (6s to 5d in cesium 11 , for example). In these cases, the liquid is denser than the solid when they coexist, possibly because of structural or electronic transitions taking place continuously in the liquid, as a function of pressure, but only at discrete pressure intervals in the solid.In contrast, recent experiments 8 have shown that hydrogen phase I -a solid structure with rotationally-free molecules associated with the sites of a hexagonal closed packed (hcp) lattice -persist...

Hydrogen-helium mixtures at conditions of Jupiter's interior are studied with first-principles computer simulations. The resulting equation of state (EOS) implies that Jupiter possesses a central core of 14 -18 Earth masses of heavier elements, a result that supports core accretion as standard model for the formation of hydrogen-rich giant planets. Our nominal model has about 2 Earth masses of planetary ices in the H-He-rich mantle, a result that is, within modeling errors, consistent with abundances measured by the 1995 Galileo Entry Probe mission (equivalent to about 5 Earth masses of planetary ices when extrapolated to the mantle), suggesting that the composition found by the probe may be representative of the entire planet. Interior models derived from this first-principles EOS do not give a match to Jupiter's gravity moment J4 unless one invokes interior differential rotation, implying that jovian interior dynamics has an observable effect on the measured gravity field.

Equilibrium properties of hydrogen-helium mixtures under conditions similar to the interior of giant gas planets are studied by means of first principle density functional molecular dynamics simulations. We investigate the molecular and atomic fluid phase of hydrogen with and without the presence of helium for densities between ρ = 0.19 g cm −3 and ρ = 0.66 g cm −3 and temperatures from T = 500 K to T = 8000 K. Helium has a crucial influence on the ionic and electronic structure of the liquid. Hydrogen molecule bonds are shortened as well as strengthened which leads to more stable hydrogen molecules compared to pure hydrogen for the same thermodynamic conditions. The ab initio treatment of the mixture enables us to investigate the validity of the widely used linear mixing approximation. We find deviations of up to 8% in energy and volume from linear mixing at constant pressure in the region of molecular dissociation.

We present a theoretical study of solid carbon dioxide (CO2) up to 50 GPa and 1500 K using firstprinciples calculations. In this pressure-temperature range, interpretations of recent experiments have suggested the existence of CO2 phases which are intermediate between molecular and covalentbonded solids. We reexamine the concept of intermediate phases in the CO2 phase diagram and propose instead molecular structures, which provide an excellent agreement with measurements.

At high pressure and temperature, the phase diagram of elemental carbon is poorly known. We present predictions of diamond and BC8 melting lines and their phase boundary in the solid phase, as obtained from first-principles calculations. Maxima are found in both melting lines, with a triple point located at Ϸ850 GPa and Ϸ7,400 K. Our results show that hot, compressed diamond is a semiconductor that undergoes metalization upon melting. In contrast, in the stability range of BC8, an insulator to metal transition is likely to occur in the solid phase. Close to the diamond͞liquid and BC8͞liquid boundaries, molten carbon is a low-coordinated metal retaining some covalent character in its bonding up to extreme pressures. Our results provide constraints on the carbon equation of state, which is of critical importance for devising models of Neptune, Uranus, and white dwarf stars, as well as of extrasolar carbon-rich planets.phase transitions ͉ melting ͉ high pressure ͉ molecular dynamics ͉ metalization E lemental carbon has been known since prehistory, and diamond is thought to have been first mined in India Ͼ2,000 years ago, although recent archaeological discoveries point at the possible existence of utensils made of diamond in China as early as 4,000 before Christ (1). Therefore, the properties of diamond and its practical and technological applications have been extensively investigated for many centuries. In the last few decades, after the seminal work of Bundy and coworkers (2) in the 1950s and '60s, widespread attention has been devoted to studying diamond under pressure (3). For example, the properties of diamond and, in general, of carbon under extreme pressure and temperature conditions are needed to devise models of outer planet interiors (e.g., Neptune and Uranus) (4-6), white dwarfs (7, 8) and extrasolar carbon planets (9).Nevertheless, under extreme conditions the phase boundaries and melting properties of elemental carbon are poorly known, and its electronic properties are not well understood. Experimental data are scarce because of difficulties in reaching megabar (1 bar ϭ 100 kPa) pressures and thousands of Kelvin regimes in the laboratory. Theoretically, sophisticated and accurate models of chemical bonding transformations under pressure are needed to describe phase boundaries. In most cases, such models cannot be simply derived from fits to existing experimental data, and one needs to resort to first-principles calculations, which may be very demanding from a computational standpoint.It has long been known that diamond is the stable phase of carbon at pressures above several gigapascal (2). Total energy calculations (10, 11) based on Density Functional Theory (DFT) predict a transition to another fourfold coordinated phase with the BC8 symmetry ¶ at Ϸ1,100 GPa and 0 K, followed by a transition to a simple cubic phase at pressures Ͼ3,000 GPa. These transitions have not yet been investigated experimentally, because the maximum pressure reached so far in diamond anvil cell experiments on carbon is 140 GP...