title: Quantum Monte Carlo study of the phase diagram of solid molecular hydrogen at extreme pressures.
abstract: Establishing the phase diagram of hydrogen is a major challenge for experimental and theoretical physics. Experiment alone cannot establish the atomic structure of solid hydrogen at high pressure, because hydrogen scatters X-rays only weakly. Instead, our understanding of the atomic structure is largely based on density functional theory (DFT). By comparing Raman spectra for low-energy structures found in DFT searches with experimental spectra, candidate atomic structures have been identified for each experimentally observed phase. Unfortunately, DFT predicts a metallic structure to be energetically favoured at a broad range of pressures up to 400 GPa, where it is known experimentally that hydrogen is non-metallic. Here we show that more advanced theoretical methods (diffusion quantum Monte Carlo calculations) find the metallic structure to be uncompetitive, and predict a phase diagram in reasonable agreement with experiment. This greatly strengthens the claim that the candidate atomic structures accurately model the experimentally observed phases.
authors.affiliation: Department of Physics, Lancaster University, Lancaster LA1 4YB, UK.
authors.forename: N D
authors.initials: ND
authors.lastname: Drummond
authors.affiliation: Theory of Condensed Matter Group, Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, UK.
authors.forename: Bartomeu
authors.initials: B
authors.lastname: Monserrat
authors.affiliation: Theory of Condensed Matter Group, Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, UK.
authors.forename: Jonathan H
authors.initials: JH
authors.lastname: Lloyd-Williams
authors.affiliation: Theory of Condensed Matter Group, Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, UK.
authors.forename: P
authors.initials: P
authors.lastname: López Ríos
authors.affiliation: Department of Physics &Astronomy, University College London, Gower Street, London WC1E 6BT, UK.
authors.forename: Chris J
authors.initials: CJ
authors.lastname: Pickard
authors.affiliation: Theory of Condensed Matter Group, Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, UK.
authors.forename: R J
authors.initials: RJ
authors.lastname: Needs