Unconventional phase III of high-pressure solid hydrogen

Abstract

We reassess the phase diagram of high-pressure solid hydrogen using mean-filed and many-body wave function based approaches to determine the nature of phase III of solid hydrogen. To discover the best candidates for the phase III, Density Functional Theory with meta-generalized-gradient approximation (meta-GGA) non-empirical strongly constrained and appropriately normed (SCAN) exchange-correlation (XC) is employed. We study eleven molecular structures with different symmetry, which are the most competitive phases, within the pressure range of 100 to 500 GPa. The SCAN phase diagram predicts that the C2/c − 24 and P 6122 − 36 structures are the best candidates for the phase III with energy difference of less than 1 meV/atom. To verify the stability of the competitive insulator structures of C2/c − 24 and P 6122 − 36, we apply the diffusion quantum Monte Carlo (DMC) to optimise the percentage of the exact exchange (α) in the trial many-body wave function. We found that the optimised α equals to 40%, and the corresponding XC functional is named PBE1x. The energy gain with respect to the conventional hybrid functional (PBE0) with α = 25% varies with density and structure. The PBE1x-DMC enthalpy-pressure phase diagram predicts that the P 6122 − 36 structure is stable up to 210 GPa where it transforms to the C2/c − 24. We predict that the phase III of high-pressure solid hydrogen is polymorphic. arXiv:1906.10854v1 [cond-mat.other]

Variational and diffusion quantum Monte Carlo calculations with the CASINO code

“…224 Azadi and Kühne performed DMC calculations in which eleven molecular hydrogen structures with different symmetries were found to be the most energetically competitive phases within the pressure range studied of 100-500 GPa, concluding that phase III may be polymorphic. 225 The electronic structure of high-pressure hydrogen is also of great interest, with much experimental effort being directed towards the discovery of metallic hydrogen. Metalization of solid hydrogen at high pressure occurs via a structural phase transition rather than band-gap closure.…”

Efficient method for grand-canonical twist averaging in quantum Monte Carlo calculations

“…36,37 Calculation of the phase diagram of hydrogen and its electronic structure under extreme conditions is a challenging subject for first-principles methods, not least because the results obtained using DFT are severely affected by the choice of exchange-correlation functional. 38,39 The limitations of DFT make DMC simulations of solid hydrogen particularly valuable, but the accuracy required is very high and controlling the DMC finite-size corrections is an important issue. This is particularly the case when DMC is used to investigate the phase diagram.…”

GW space-time method: Energy band gap of solid hydrogen

“…From the theoretical perspective, which metallization scenario is realized largely depends on the specific crystal structure assumed in the calculation. At the same time, despite an extensive amount of work by experiment [8][9][10][11][12][13][14][15][16][17][18] and theory [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36], there is no definitive conclusion on the crystal structure of solid hydrogen in the range of pressures where any of the metallization scenarios might take place. It was recently observed in x-ray diffraction measurements [37] that, at least up to 250 GPa, solid hydrogen remains in the hexagonal-closed-pack (hcp) crystal form, albeit possibly with an increasingly under compression anisotropy.…”