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.
Highlights
Establishing the phase diagram of hydrogen is a major challenge for experimental and theoretical physics
We have modelled phase II using a molecular structure of P21/c symmetry with 24 atoms in the primitive unit cell, which we refer to as P21/c-24; see Fig. 1a. (We adopt the convention of labelling structures by their symmetry followed by the number of atoms per primitive cell.) P21/c-24 is the most stable structure found to date in static-lattice density functional theory (DFT) within the pressure range appropriate for phase II, and its vibrational characteristics are compatible with those of phase II
We find that the use of diffusion quantum Monte Carlo (DMC) renders the metallic Cmca-4 structure that is favoured in DFT energetically uncompetitive, leaving us with a phase diagram in reasonable quantitative agreement with experiment
Summary
Establishing the phase diagram of hydrogen is a major challenge for experimental and theoretical physics. 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. We model phase III using a C2/c-24 structure consisting of layers of molecules whose bonds lie within the planes of the layers, forming a distorted hexagonal pattern[26]; see Fig. 1b This very stable structure can account for the high IR activity of phase III26. We find that the use of DMC (and to a lesser extent the treatment of phonon anharmonicity) renders the metallic Cmca-4 structure that is favoured in DFT energetically uncompetitive, leaving us with a phase diagram in reasonable quantitative agreement with experiment
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