Thermodynamic anomalies and three distinct liquid-liquid transitions in warm dense liquid hydrogen

Abstract

The properties of hydrogen at high pressure have wide implications in astrophysics and high-pressure physics. Its phase change in the liquid is variously described as a metallization, H2-dissociation, density discontinuity or plasma phase transition. It has been tacitly assumed that these phenomena coincide at a first-order liquid-liquid transition (LLT). In this work, the relevant pressure-temperature conditions are thoroughly explored with first-principles molecular dynamics. We show there is a large dependency on exchange-correlation functional and significant finite size effects. We use hysteresis in a number of measurable quantities to demonstrate a first-order transition up to a critical point, above which molecular and atomic liquids are indistinguishable. At higher temperature beyond the critical point, H2-dissociation becomes a smooth cross-over in the supercritical region that can be modelled by a pseudo-transition, where the H2→2H transformation is localized and does not cause a density discontinuity at metallization. Thermodynamic anomalies and counter-intuitive transport behavior of protons are also discovered even far beyond the critical point, making this dissociative transition highly relevant to the Supplementary Material. Results of dimer-dimer bond-length distribution (DDLD) calculations, comparison between PBE and vdW-DF results, the thermodynamic modelling of pseudo-transition and phase boundaries, calculated isotherms (equation of state, EOS), transient clusters and their lifetime and charge state, the angular distribution function of H3 clusters, mixing of H and H2 liquid, thermal fluctuation analysis, isotope effect, finite size effects, and the assessment of proton selfdiffusivity and viscosity.

Understanding high pressure molecular hydrogen with a hierarchical machine-learned potential

“…The higher density of the liquid is most apparent in the molecule-molecule RDF. These RDFs are on good agreement with the RDFs obtained from AIMD simulations 45 . The corresponding RDF indicates that the centre of each molecule remains close to the hcp lattice sites.…”

Fully consistent density functional theory determination of the insulator-metal transition boundary in warm dense hydrogen

“…The inclusion of rVV10 tends to lower the boundary position by ≈150 K, with little to no effect outside 100 P 200 GPa. The relative shift is significantly smaller than the difference between any two XC functionals [13,[20][21][22][23]. That suggests deficiencies in the description of the short-and intermediate-range interactions across a wide range of PT space as the cause for the variety of predictions.…”

“…It is known that PBE underestimates the metallization pressure while vdW-DF1 overbinds H 2 molecules and hence delays dissociation (see Ref. [23]). Due to the sensitivity of the IMT to the underlying sampling of the ion configuration space, a more accurate XC functional for this system should provide structural properties that lie within those bounds.…”

“…However, the XC free-energy functional must be approximated [18,19]. Some of the earlier (and a few more recent) H IMT predictions from DFT were done in the Born-Oppenheimer (BO) approximation [20][21][22][23][24][25] and employed the ground-state Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) XC functional [26]. Those results demonstrated no more than ≈25 GPa pressure difference from earlier SCEs [7][8][9], an IMT phase boundary in the lower-pressure liquid regime.…”

“…The incorporation of van der Waals (vdW) interactions [13], via the vdW-DF1 [30] and vdW-DF2 [31] XC functionals, shifted the predicted IMT boundary substantially up, by 100 and 200 GPa, respectively. Recently, Geng et al assumed pragmatically that the IMT boundary lies between the values from the PBE and vdW-DF1 XC functionals [23]. Though plausible, the separation is uncontrolled and large, especially for vdW-DF2 vs PBE, once NQEs are included; see Ref.…”