Phase boundary of hot dense fluid hydrogen

Ohta, Kenji; Ichimaru, Kota; Einaga, Mari; Kawaguchi, Sho; Shimizu, Katsuya; Matsuoka, Toshifumi; Hirao, Naohisa; Ohishi, Yasuo

doi:10.1038/srep16560

Cited by 84 publications

(126 citation statements)

References 31 publications

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“…Ohta et al [68] studied hydrogen in a DAC using cw laser heating with gold as an absorber and found anomalies in heating curves consistent with the P /T points for plateaus in the study of Dzyabura et al [69] and our studies here. Recently, Knudson et al published a paper on metallization of deuterium using reverberating shock and ramp compression [70].…”

Section: Discussionsupporting

confidence: 61%

Evidence of a first-order phase transition to metallic hydrogen

Zaghoo¹,

Salamat²,

Silvera³

2016

Phys. Rev. B

141

166

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Section: Discussionsupporting

confidence: 61%

Evidence of a first-order phase transition to metallic hydrogen

Zaghoo¹,

Salamat²,

Silvera³

2016

Phys. Rev. B

141

166

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“…This evidence has been qualitatively confirmed also by Ref. 33, although this experiment focus on larger temperatures. In a second experiment, Knudson and coworkers [34] observed the IMT in deuterium at much larger pressures instead.…”

supporting

confidence: 74%

Acceleratingab initioMolecular Dynamics and Probing the Weak Dispersive Forces in Dense Liquid Hydrogen

Mazzola

Sorella

2017

Phys. Rev. Lett.

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We propose an ab-initio molecular dynamics method, capable to reduce dramatically the autocorrelation time required for the simulation of classical and quantum particles at finite temperature. The method is based on an efficient implementation of a first order Langevin dynamics modified by means of a suitable, position dependent acceleration matrix S. Here we apply this technique, within a Quantum Monte Carlo (QMC) based wavefunction approach and within the Born-Oppheneimer approximation, for determining the phase diagram of high-pressure Hydrogen with simulations much longer than the autocorrelation time. With the proposed method, we are able to equilibrate in few hundreds steps even close to the liquid-liquid phase transition (LLT). Within our approach we find that the LLT transition is consistent with recent density functionals predicting a much larger transition pressures when the long range dispersive forces are taken into account.One of the most important problems in ab-initio molecular dynamics (MD) is the coexistence of much different time scales underlying physical processes, even when computing equilibrium finite temperature properties. This has been a challenge for most ab-initio simulations in systems of biophysical interest, the most striking one being protein folding [1,2], where the microscopic time scale of molecular vibration is of the order of f s, while the macroscopic time scale of the folding exceeds the µs. Also in water system simulations, the weak Van der Waals (vdW) interactions and the related hydrogen bond imply very difficult equilibration properties at ambient conditions, so that the most accurate simulations are based on force field fitting forces. We will show here that, a computationally expensive ab-initio method for dense liquid hydrogen can be combined with a sampling technique capable to reduce drastically the autocorrelation times. This method has the potential, due to its simplicity, to be applied with success also to much more complex systems and other ab-initio techniques.At variance of all previous attempts[3-10], we propose that an optimal way to get rid of different time scales is based on the use of first order Langevin dynamics:where T is the temperature, f Ri = −∂ Ri V ( R) and V R is an energy potential of the classical coordinates R, e.g. atomic positions. For S ij = δ ij this is the conventional first order Langevin dynamics (CFOLD). Preconditioned Langevin equations, formally similar to Eq. 40, have been already introduced in several fields different from molecular dynamics, such as gauge field theories[11], statistics [12] and machine learning [13], with different definitions of the preconditioning matrix S. Instead, the main idea of this work is based on the simple solution of CFOLD for the harmonic potential V (R) = −K R, where K is the elastic matrix term. In this case one is able to show that the i) the autocorrelation time is independent of temperature τ −1 corr = K min , where K min (K max ) is the lowest (largest) non-zero eigenvalue of the elastic matrix...

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“…When experiments achieved the predicted transition pressure, they did not find a metallic state but a rich phase diagram with several different molecular structures [3][4][5]. The quest for metallic hydrogen at low temperature is still an on-going activity with different experimental methods providing conflicting results [6][7][8][9][10].…”

Section: Introductionmentioning

confidence: 99%

Coupled electron-ion Monte Carlo simulation of hydrogen molecular crystals

Rillo

Morales

Ceperley

et al. 2017

The Journal of Chemical Physics

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We performed simulations for solid molecular hydrogen at high pressures (250GPa≤P≤500GPa) along two isotherms at T=200 K (phases III and VI) and at T=414 K (phase IV). At T=200K we considered likely candidates for phase III, the C2c and Cmca12 structures, while at T=414K in phase IV we studied the Pc48 structure. We employed both Coupled Electron-Ion Monte Carlo (CEIMC) and Path Integral Molecular Dynamics (PIMD) based on Density Functional Theory (DFT) using the vdW-DF approximation. The comparison between the two methods allows us to address the question of the accuracy of the xc approximation of DFT for thermal and quantum protons without recurring to perturbation theories. In general, we find that atomic and molecular fluctuations in PIMD are larger than in CEIMC which suggests that the potential energy surface from vdW-DF is less structured than the one from Quantum Monte Carlo. We find qualitatively different behaviors for systems prepared in the C2c structure for increasing pressure. Within PIMD the C2c structure is dynamically partially stable for P≤250GPa only: it retains the symmetry of the molecular centers but not the molecular orientation; at intermediate pressures it develops layered structures like Pbcn or Ibam and transforms to the metallic Cmca-4 structure at P≥450GPa. Instead, within CEIMC, the C2c structure is found to be dynamically stable at least up to 450GPa; at increasing pressure the molecular bond length increases and the nuclear correlation decreases. For the other two structures the two methods are in qualitative agreement although quantitative differences remain. We discuss various structural properties and the electrical conductivity. We find these structures become conducting around 350GPa but the metallic Drude-like behavior is reached only at around 500GPa, consistent with recent experimental claims.

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Phase boundary of hot dense fluid hydrogen

Cited by 84 publications

References 31 publications

Evidence of a first-order phase transition to metallic hydrogen

Evidence of a first-order phase transition to metallic hydrogen

Acceleratingab initioMolecular Dynamics and Probing the Weak Dispersive Forces in Dense Liquid Hydrogen

Coupled electron-ion Monte Carlo simulation of hydrogen molecular crystals

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