Critical point and mechanism of the fluid–fluid phase transition in warm dense hydrogen

Norman, G. É.; Saitov, I. M.

doi:10.1134/s1028335817060088

Cited by 15 publications

(24 citation statements)

References 12 publications

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“…In this paper, we have presented a more extensive analysis of the local nuclear structure around the transition line, providing data for the proton-proton pair correlation functions and a cluster analysis to determine the formation of other aggregates in the dissociated phase recently proposed. [18,19] We indeed confirm the abrupt character of the transition on structural properties other than the EOS and the g(r) but do not find evidence of the formation of stable aggregates such as H + 3 . At the highest temperature studied, T = 3,000 K, no discontinuity is observed in the EOS and in the g(r), showing a continuous dissociation.…”

Section: Figuresupporting

confidence: 51%

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Local structure in dense hydrogen at the liquid–liquid phase transition by coupled electron–ion Monte Carlo

Pierleoni

Holzmann

Ceperley

2017

Contributions to Plasma Physics

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We present a study of the local structure of high‐pressure hydrogen around the liquid–liquid transition line based on results from the coupled electron–ion Monte Carlo method. We report results for the equation of state and for the radial distribution function between protons g(r) and results from a cluster analysis to detect the possible formation of stable molecular ions beyond the transition line as well as above the critical temperature. We discuss various estimates for the molecular fraction in both phases and show that, although the presence of H3+ ions is suggested by the form of the g(r), they are not stable against thermal fluctuations.

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Section: Figuresupporting

confidence: 51%

“…Recently, Norman and Saitov used BOMD with classical protons and the PBE functional to study the same phase transition. Based on the comparative analysis of the g ( r )s of the two coexisting densities, they proposed that in the fluid just beyond the transition line, the molecular phase transforms to a mixture of H + ,

H_{3}^{+}

, and

H_{2}^{+}

.…”

Section: Cluster Analysismentioning

confidence: 99%

Local structure in dense hydrogen at the liquid–liquid phase transition by coupled electron–ion Monte Carlo

Pierleoni

Holzmann

Ceperley

2017

Contributions to Plasma Physics

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“…16 functional on a 128 particle system, we use a 3x3x3 Monkhorst-Pack k-point sampling We denote with ∆g(r) the difference between the two g(r)'s corresponding to the densities (indicated in the figure legend) closest to the phase transition, before and after it 26 . The change in the liquid structure is more evident at low temperature (1200 K).…”

Section: Dft Molecular Dynamics Setupmentioning

confidence: 99%

“…2). Although a finer mesh of densities is required, as well as an extended finite size scaling in order to precisely resolve the existence of the plateau in the EOS, the observed feature suggests the existence of a critical point above the previously expected temperature of 1500-2000 K 21,35 (with the notable exception of Norman et al 26 who predict a critical temperature of 4000 K from PBE simulations).…”

mentioning

confidence: 99%

Phase Diagram of Hydrogen and a Hydrogen-Helium Mixture at Planetary Conditions by Quantum Monte Carlo Simulations

2018

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Understanding planetary interiors is directly linked to our ability of simulating exotic quantum mechanical systems such as hydrogen (H) and hydrogen-helium (H-He) mixtures at high pressures and temperatures 1 . Equations of State (EOSs) tables based on Density Functional Theory (DFT), are commonly used by planetary scientists, although this method allows only for a qualitative description of the phase diagram 2 , due to an incomplete treatment of electronic interactions 3 . Here we report Quantum Monte Carlo (QMC) molecular dynamics simulations of pure H and H-He mixture. We calculate the first QMC EOS at 6000 K for an H-He mixture of a proto-solar composition, and show the crucial influence of He on the H metallization pressure. Our results can be used to calibrate other EOS calculations and are very timely given the accurate determination of Jupiter's gravitational field from the NASA Juno mission and the effort to determine its structure 4 .Since a few decades the link between the uncertainty of the H EOS and the internal structure of Jupiter (and other gaseous planets) has been investigated and many efforts to model Jupiter's interior have been carried 1,5-7 . The computation of an EOS from first principles requires to solve a many-body quantum mechanical problem, a task which is beyond the currently available theoretical and computational capabilities. In practice, we must resort to several approximations. The first is to decouple the ionic and electronic problems and consider the ions as classical or quantum particles, determining their motion by following the Born-Oppenheimer potential energy surface. The second approximation concerns the description of the electronic interaction and the exchange one, due to the Pauli exclusion principle.The standard approach to EOS calculations relies on Density Functional Theory (DFT), which targets the tridimensional electronic density rather than the (N e electrons) many-body wave-function. Its success and simplicity have lead to a widespread application in materials science and to the development of several software packages which allow fast and reproducible calculations 8 . Although DFT is formally exact, the explicit functional form to describe the exchange and correlation (XC) effects between electrons remains approximated 3 . Indeed, a systematic and efficient route to improve XC functional is still lacking. Therefore, in practical solid state calculations, benchmarks against experimental data, are often required to validate the XC functional used to describe the system in a satisfactory manner.Dense hydrogen systems, both in the low temperature solid and in the liquid phase remain a challenge to DFT simulations due to the interplay of strong correlation and non-covalent interactions between the atoms. DFT calculations with different functionals produce different results with the expected metallization pressure varying over a range of 100-200 GPa (Fig. 1) 12,13 . Since experimental data are limited, it is not possible to identify a posteriori the best functional...

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“…The objective of this paper is to elucidate the nature of the phase transition in warm dense hydrogen. Preliminary results have already been published …”

Section: Introductionmentioning

confidence: 99%

Plasma phase transition in warm dense hydrogen

Norman

Saitov

2018

Contributions to Plasma Physics

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A new idea is developed that the fluid–fluid phase transition in warm dense hydrogen is related to the partial ionization of molecular hydrogen H2 with formation of molecular ions H2+ and H3+. Conventional ab initio quantum modelling is applied. The proton–proton pair correlation functions (PCFs) obtained are used for the non‐conventional diagnostics of the phase transition and elucidation of its nature for temperatures in the range 700–1500 K. Short‐ and long‐range changes of PCFs are studied. Ionization of H2 molecules and the appearance of molecular ions H2+ and H3+ are revealed. The validity of the soft sphere model is tested for the long‐range order.

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Critical point and mechanism of the fluid–fluid phase transition in warm dense hydrogen

Cited by 15 publications

References 12 publications

Local structure in dense hydrogen at the liquid–liquid phase transition by coupled electron–ion Monte Carlo

Local structure in dense hydrogen at the liquid–liquid phase transition by coupled electron–ion Monte Carlo

Phase Diagram of Hydrogen and a Hydrogen-Helium Mixture at Planetary Conditions by Quantum Monte Carlo Simulations

Plasma phase transition in warm dense hydrogen

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