Quantum Monte Carlo simulations of high pressure solid hydrogen

Several solid phases of hydrogen have been observed but the stable structures of hydrogen at high pressure remain experimentally undetermined because of the weak scattering of protons in x-ray diffraction studies. Theoretical identification of the structures is also difficult because of the small energy differences between competing phases and the large zero-point (ZP) contributions to the energies. We have performed static-nucleus diffusion Monte Carlo calculations for the candidate high pressure phases across a range of relevant densities and added ZP energies from both harmonic and anharmonic density functional theory calculations. We have used our calculated total energies to construct an enthalpy-pressure phase diagram from which we have evaluated the relative stability of the candidate structures. \\[4pt] This work was facilitated by a 2013 INCITE award of computing resources on Titan at Oak Ridge National Laboratory. It also made use of facilities provided by the High Performance Computing Service at the University of Cambridge and the N8 High Performance Computing Service which is coordinated by the Universities of Leeds and Manchester. Financial support was provided by the Engineering and Physical Sciences Research Council (UK).