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Quantum Monte Carlo study of the phase diagram of solid molecular hydrogen at extreme pressures.

Drummond ND, Monserrat B, Lloyd-Williams JH, López Ríos P, Pickard CJ, Needs RJ - Nat Commun (2015)

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, Lancaster University, Lancaster LA1 4YB, UK.

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.

No MeSH data available.


DFT and DMC static-lattice enthalpy–pressure relations for the different H structures relative to C2/c-24.(a) DFT–PBE, (b) DFT–BLYP and (c) DMC. The relative DFT enthalpies are converged to better than 0.1 meV per atom. The widths of the DMC lines indicate the estimated uncertainties in the enthalpies due to single-particle finite-size errors, which are greater than the uncertainties due to random sampling in the Monte Carlo algorithm, as explained in Supplementary Note 2.
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f2: DFT and DMC static-lattice enthalpy–pressure relations for the different H structures relative to C2/c-24.(a) DFT–PBE, (b) DFT–BLYP and (c) DMC. The relative DFT enthalpies are converged to better than 0.1 meV per atom. The widths of the DMC lines indicate the estimated uncertainties in the enthalpies due to single-particle finite-size errors, which are greater than the uncertainties due to random sampling in the Monte Carlo algorithm, as explained in Supplementary Note 2.

Mentions: Figure 2 shows the static-lattice enthalpies of the structures relative to C2/c-24. In Fig. 2a,b we report DFT enthalpies calculated using the Perdew-Burke-Ernzerhof (PBE)42 and Becke–Lee–Yang–Parr (BLYP) density functionals4344. The relative DFT enthalpies are converged to better than 0.1 meV per atom with respect to k-point sampling and plane wave cutoff energy. The difference between the DFT–PBE and DFT–BLYP relative enthalpies arises chiefly from the energetics and not from the slightly different structures obtained from geometry optimization calculations performed at fixed external pressures using the two different functionals: see Supplementary Note 1 and the accompanying Supplementary Fig. 1. In Fig. 2c, we report DMC enthalpies, which were obtained by fitting polynomials to the extrapolated infinite-system-size DMC energies as a function of volume, and differentiating the polynomials to obtain pressures. The structures used for the DMC calculations were obtained from DFT–PBE geometry optimization calculations. We truncate the DMC enthalpy curves at the highest and lowest pressures at which we have performed calculations.


Quantum Monte Carlo study of the phase diagram of solid molecular hydrogen at extreme pressures.

Drummond ND, Monserrat B, Lloyd-Williams JH, López Ríos P, Pickard CJ, Needs RJ - Nat Commun (2015)

DFT and DMC static-lattice enthalpy–pressure relations for the different H structures relative to C2/c-24.(a) DFT–PBE, (b) DFT–BLYP and (c) DMC. The relative DFT enthalpies are converged to better than 0.1 meV per atom. The widths of the DMC lines indicate the estimated uncertainties in the enthalpies due to single-particle finite-size errors, which are greater than the uncertainties due to random sampling in the Monte Carlo algorithm, as explained in Supplementary Note 2.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4525154&req=5

f2: DFT and DMC static-lattice enthalpy–pressure relations for the different H structures relative to C2/c-24.(a) DFT–PBE, (b) DFT–BLYP and (c) DMC. The relative DFT enthalpies are converged to better than 0.1 meV per atom. The widths of the DMC lines indicate the estimated uncertainties in the enthalpies due to single-particle finite-size errors, which are greater than the uncertainties due to random sampling in the Monte Carlo algorithm, as explained in Supplementary Note 2.
Mentions: Figure 2 shows the static-lattice enthalpies of the structures relative to C2/c-24. In Fig. 2a,b we report DFT enthalpies calculated using the Perdew-Burke-Ernzerhof (PBE)42 and Becke–Lee–Yang–Parr (BLYP) density functionals4344. The relative DFT enthalpies are converged to better than 0.1 meV per atom with respect to k-point sampling and plane wave cutoff energy. The difference between the DFT–PBE and DFT–BLYP relative enthalpies arises chiefly from the energetics and not from the slightly different structures obtained from geometry optimization calculations performed at fixed external pressures using the two different functionals: see Supplementary Note 1 and the accompanying Supplementary Fig. 1. In Fig. 2c, we report DMC enthalpies, which were obtained by fitting polynomials to the extrapolated infinite-system-size DMC energies as a function of volume, and differentiating the polynomials to obtain pressures. The structures used for the DMC calculations were obtained from DFT–PBE geometry optimization calculations. We truncate the DMC enthalpy curves at the highest and lowest pressures at which we have performed calculations.

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, Lancaster University, Lancaster LA1 4YB, UK.

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.

No MeSH data available.