EVIDENCE FROM MOLECULAR-DYNAMICS SIMULATIONS FOR NONMETALLIC BEHAVIOROF SOLID HYDROGEN ABOVE 160 GPA

THE behaviour of molecular hydrogen at high pressures has implications for the interiors of the giant planets, which consist mainly of hydro gen, In particular, the question of whether solid hydrogen becomes met allic under these conditions has been much debated(1-9), in part becau se the structure that molecular hydrogen adopts at high pressure is no t known, Here we report the results of first-principles molecular dyna mics simulations of solid hydrogen at pressures up to 270 GPa. We find that at 77 K, hydrogen exists as a stable, orientationally disordered phase up to 60 GPa, consistent with experimental results(1,10). As th e presssure is raised, a gradual transformation to an ordered orthorho mbic structure begins at 160 GPa, and by 260 GPa the solid becomes sem iconducting, with an indirect band gap of 1.4 eV. The calculated vibra tional density of states of this phase is consistent with infrared and Raman spectra measured up to 160 GPa (ref. 11). Although limitations on the simulation time and size may result in an overestimate of the a bsolute pressure, our calculations show that solid hydrogen does not b ecome metallic, even at pressures approaching 260 GPa.