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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–PBE vibrational contributions to the enthalpies of the H structures.(a) Harmonic zero-point (ZP) contributions to enthalpies, (b) anharmonic ZP corrections to enthalpies, (c) harmonic ZP enthalpies relative to C2/c-24, and (d) anharmonic ZP corrections relative to C2/c-24. P21/c-24 is destabilized by both harmonic vibrations and anharmonic corrections, relative to C2/c-24. Cmca-12, Cmca-4, and Pc-48 are all stabilised by harmonic vibrations but destabilized by anharmonic corrections, relative to C2/c-24.
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f3: DFT–PBE vibrational contributions to the enthalpies of the H structures.(a) Harmonic zero-point (ZP) contributions to enthalpies, (b) anharmonic ZP corrections to enthalpies, (c) harmonic ZP enthalpies relative to C2/c-24, and (d) anharmonic ZP corrections relative to C2/c-24. P21/c-24 is destabilized by both harmonic vibrations and anharmonic corrections, relative to C2/c-24. Cmca-12, Cmca-4, and Pc-48 are all stabilised by harmonic vibrations but destabilized by anharmonic corrections, relative to C2/c-24.

Mentions: The harmonic zero-point contributions to the enthalpies of the H phases increase sublinearly with pressure, as shown in Fig. 3a, while the anharmonic corrections tend to decrease with pressure; see Fig. 3b. The harmonic zero-point enthalpies are roughly 30 times larger than the anharmonic corrections. However, the differences between the harmonic zero-point energies of the five phases considered at fixed pressure are similar in magnitude to the differences between the anharmonic corrections, both being about 10 meV per atom, as shown in Fig. 3c,d. This demonstrates that the variations in the anharmonic vibrational corrections are as important as those of the harmonic contributions to the enthalpies in determining the relative stabilities of phases in this system.


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–PBE vibrational contributions to the enthalpies of the H structures.(a) Harmonic zero-point (ZP) contributions to enthalpies, (b) anharmonic ZP corrections to enthalpies, (c) harmonic ZP enthalpies relative to C2/c-24, and (d) anharmonic ZP corrections relative to C2/c-24. P21/c-24 is destabilized by both harmonic vibrations and anharmonic corrections, relative to C2/c-24. Cmca-12, Cmca-4, and Pc-48 are all stabilised by harmonic vibrations but destabilized by anharmonic corrections, relative to C2/c-24.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: DFT–PBE vibrational contributions to the enthalpies of the H structures.(a) Harmonic zero-point (ZP) contributions to enthalpies, (b) anharmonic ZP corrections to enthalpies, (c) harmonic ZP enthalpies relative to C2/c-24, and (d) anharmonic ZP corrections relative to C2/c-24. P21/c-24 is destabilized by both harmonic vibrations and anharmonic corrections, relative to C2/c-24. Cmca-12, Cmca-4, and Pc-48 are all stabilised by harmonic vibrations but destabilized by anharmonic corrections, relative to C2/c-24.
Mentions: The harmonic zero-point contributions to the enthalpies of the H phases increase sublinearly with pressure, as shown in Fig. 3a, while the anharmonic corrections tend to decrease with pressure; see Fig. 3b. The harmonic zero-point enthalpies are roughly 30 times larger than the anharmonic corrections. However, the differences between the harmonic zero-point energies of the five phases considered at fixed pressure are similar in magnitude to the differences between the anharmonic corrections, both being about 10 meV per atom, as shown in Fig. 3c,d. This demonstrates that the variations in the anharmonic vibrational corrections are as important as those of the harmonic contributions to the enthalpies in determining the relative stabilities of phases in this system.

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.