Miscibility and Transport Properties in Hydrogen-Neon Mixtures

Arianna Glaeson; Armin Bergermann; Ronald Redmer; Siegfried Glenzer

arxiv: 2604.10355 · v1 · submitted 2026-04-11 · 🌌 astro-ph.EP · cond-mat.mtrl-sci

Miscibility and Transport Properties in Hydrogen-Neon Mixtures

Armin Bergermann , Siegfried Glenzer , Arianna Glaeson , Ronald Redmer This is my paper

Pith reviewed 2026-05-10 15:08 UTC · model grok-4.3

classification 🌌 astro-ph.EP cond-mat.mtrl-sci

keywords hydrogen-neon mixturesphase separationdensity functional theorymolecular dynamicsplanetary interiorselectrical conductivitygiant planetsmiscibility

0 comments

The pith

Hydrogen-neon mixtures phase-separate at substantially lower pressures than hydrogen-helium mixtures, with neon stabilizing molecules and slashing electrical conductivity.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper applies density functional theory molecular dynamics to hydrogen mixed with neon under the extreme pressures and temperatures inside giant planets. It finds that neon causes the mixture to separate into distinct phases at lower pressures than helium does in similar conditions. Neon also keeps hydrogen molecules from breaking apart even at 10000 K and 10 Mbar, which cuts the electrical conductivity by several orders of magnitude relative to pure hydrogen. Because neon scatters X-rays more strongly than helium, the mixture offers a practical laboratory proxy for observing these separation processes directly.

Core claim

Density functional theory molecular dynamics simulations show that the minimum pressure required to trigger phase separation in hydrogen-neon mixtures is substantially lower than in hydrogen-helium mixtures. The presence of neon stabilizes hydrogen molecules even at temperatures of 10000 K and pressures of 10 Mbar, an effect that is significantly more pronounced than in hydrogen-helium mixtures and is accompanied by a reduction of several orders of magnitude in the electrical conductivity compared to pure hydrogen.

What carries the argument

Density functional theory combined with molecular dynamics simulations that track molecular stability, phase boundaries, and electrical conductivity in hydrogen-neon mixtures at planetary interior conditions.

If this is right

Interior models of Jupiter and Saturn must account for phase separation occurring at lower pressures when neon is present.
Hydrogen-neon mixtures become a viable experimental surrogate for studying hydrogen-rich phase separation because neon has a larger X-ray scattering cross section.
Electrical conductivity in planetary interiors is reduced when heavier elements stabilize molecular hydrogen.
Physical mechanisms governing mixing and transport in hydrogen with heavier elements can be isolated and tested more readily than with helium.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

Neon may influence convection and heat flow patterns inside giant planets more strongly than previously modeled.
Similar stabilization effects could appear with other trace heavy elements and might alter predictions for magnetic field generation.
Laboratory X-ray scattering experiments on compressed hydrogen-neon samples could directly test the simulated conductivity and phase behavior.
The lower separation pressure suggests that trace neon could affect the depth at which immiscibility layers form in real planetary interiors.

Load-bearing premise

The chosen exchange-correlation functional and molecular dynamics setup accurately capture the electronic structure, molecular stability, and phase behavior of hydrogen-neon mixtures at 10 Mbar and 10000 K without large systematic errors.

What would settle it

A direct measurement of the pressure at which phase separation begins in a hydrogen-neon mixture near 10000 K, or a conductivity measurement showing the predicted orders-of-magnitude drop relative to pure hydrogen.

Figures

Figures reproduced from arXiv: 2604.10355 by Arianna Glaeson, Armin Bergermann, Ronald Redmer, Siegfried Glenzer.

**Figure 1.** Figure 1: Pair distribution function of H-H (red), H-Ne (black), and Ne-Ne (blue) for a temperature of 10000 K and a pressure of 11.5 Mbar. The inset shows a typical snapshot of the 320 H (red) and 192 Ne (blue) atoms. 3. RESULTS To characterize the microscopic structure and ionic transport properties of H–Ne mixtures, we analyze pair distribution functions (Sec. 3.1), molecular bond lifetimes (Sec. 3.2), and self… view at source ↗

**Figure 4.** Figure 4: Lifetimes of H2 molecules at 10000 K. Panel a) shows results for different concentrations and a pressure of ≈ 6 Mbar. Panel b) depicts the lifetimes of H2 molecules for a H concentration of xH = 0.125 and different pressures (color-coded). shorter than the equilibrium bond length of an isolated H2 molecule (≈ 0.74 Å) (Syrkin & Dyatkina (1964)). Additionally, we present a typical snapshot of our simulatio… view at source ↗

**Figure 3.** Figure 3: Pair distribution functions gH-Ne(r) for xH = 0.625, various pressures (color-coded) and a temperature of 10000 K. Panel a) shows the H-H and b) shows the H-Ne PDF. Additionally, we present a typical snapshot of the simulation cell as an inset. at p–T conditions far above the well-known liquid–liquid transition for pure H (Bergermann et al. (2024b); Lorenzen et al. (2010); Hinz et al. (2020); Scandolo (20… view at source ↗

**Figure 5.** Figure 5: Self-diffusion coefficients of H and Ne at T = 10,000 K as a function of pressure for mixtures with xH = 0.125, 0.500, and 0.875, obtained from mean-square displacement analysis [PITH_FULL_IMAGE:figures/full_fig_p005_5.png] view at source ↗

**Figure 6.** Figure 6: shows the miscibility diagram obtained in this study. Each marker corresponds to a specific p–T condition at which the height of the first peak in the H–Ne PDF reaches its maximum. The solid colored lines connect these points for each composition, and phase separation is predicted to occur below these lines, where the color indicates the corresponding H concentration. At temperatures below 10000 K, phase … view at source ↗

**Figure 7.** Figure 7: Electrical conductivity of H-Ne mixtures for a temperature of 10000 K. The 7 different xH concentrations are color-coded. The gray dashed line depicts the minimum metallic conductivity as derived from the Mott criterion for T = 0 K, the gray dotted line indicates the corresponding value for fluid H and fluid alkali metals at finite temperatures, and the black dashed line with crosses outlines the pressur… view at source ↗

**Figure 8.** Figure 8: Thermal conductivity of H-Ne mixtures for a temperature of 10000 K. The 7 different xH concentrations are color-coded. The black dashed line with crosses outline the pressure where the individual xH concentrations start to phase separate. lead to a much smaller Brillouin zone and a correspondingly larger plane-wave basis set. In addition, ionic diffusion—particularly of the heavier Ne atoms—becomes slow… view at source ↗

**Figure 9.** Figure 9: PDFs at a temperature of T = 10000 K and a H concentration xH = 0.125 with 512 particles. Panel (a) displays the H–H PDF, while panel (b) shows the H–Ne PDF. Different pressures are distinguished by color coding. markedly above 1 at large separations, while the probability of finding H–H is suppressed (below 1). Note that the heights of the first H–Ne peaks and the long-range tails signal the onset of ph… view at source ↗

**Figure 12.** Figure 12: PDFs at a temperature of 10000 K and a H concentration xH = 0.875 with 512 particles. Panel (a) displays the H–H PDF, while panel (b) shows the H–Ne PDF. Different pressures are distinguished by color coding. sistent with the sharper criterion provided by the H–Ne peak height. Finally, we show the PDFs for H–H and H–Ne at a higher H concentration of xH = 0.875 ( [PITH_FULL_IMAGE:figures/full_fig_p009_12.png] view at source ↗

**Figure 11.** Figure 11: PDFs at a temperature of 10000 K and a H concentration xH = 0.625 with 512 particles. Panel (a) displays the H–H PDF, while panel (b) shows the H–Ne PDF. Different pressures are distinguished by color coding. estimate as the pressure where H and Ne begin to lose miscibility. At higher pressures, the tails in the PDFs again display phase separation signatures: the probability of finding H–Ne pairs incre… view at source ↗

**Figure 13.** Figure 13: Height of the first peak in the PDF for 128, 256, and 512 particles for all p-T-x conditions investigated in this work. The dashed line shows the pressure corresponding to the estimated onset of phase separation [PITH_FULL_IMAGE:figures/full_fig_p011_13.png] view at source ↗

read the original abstract

The mixing behavior of hydrogen with heavier elements plays a key role in modeling the interiors of giant planets such as Jupiter and Saturn. Using density functional theory combined with molecular dynamics, we investigate hydrogen-neon mixtures and find that the minimum pressure required to trigger phase separation is substantially lower than in hydrogen-helium mixtures. Our simulations further reveal that the presence of neon stabilizes hydrogen molecules even at temperatures of 10000 K and pressures of 10 Mbar, similar to trends observed in hydrogen-helium mixtures but significantly more pronounced. This stabilization is accompanied by a reduction of several orders of magnitude in the electrical conductivity compared to pure hydrogen. These results, together with the larger X-ray scattering cross section of neon, establish hydrogen-neon as a valuable experimental surrogate for probing phase separation in hydrogen-rich mixtures and provide new insight into the physical mechanisms in hydrogen and mixtures with heavier elements under planetary interior conditions

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

H-Ne DFT-MD runs show lower phase-separation pressure and stronger H2 stabilization than H-He, but the numbers rest on an unbenchmarked functional at 10 Mbar and 10,000 K.

read the letter

The main point is that neon lowers the pressure for phase separation in hydrogen mixtures relative to helium, keeps H2 molecules stable farther into the 10,000 K and 10 Mbar regime, and cuts electrical conductivity by orders of magnitude. The authors also note neon's larger X-ray cross section makes it a convenient lab stand-in for testing the same physics. These are concrete, new quantitative claims for the neon system that extend the earlier H-He literature they cite, and they give modelers of Jupiter and Saturn something specific to try in their equations of state. The surrogate suggestion is practical and worth testing experimentally. That part of the work is useful on its own terms. The soft spot is exactly the one flagged in the stress test. All headline results come from DFT-MD trajectories, and at these conditions the location of the molecular-to-atomic transition and the miscibility gap are known to move by several Mbar depending on the exchange-correlation functional. The paper does not appear to include direct comparisons to QMC or hybrid-functional benchmarks for the H-Ne mixture, nor does the abstract report the functional, cell sizes, or convergence checks. If those details and some validation runs are in the full text, the quantitative thresholds become more credible; otherwise the exact pressure and conductivity numbers stay provisional. The overall approach is standard for the field and the qualitative trends line up with chemical expectations, so there is no internal contradiction. This paper is for planetary-interior modelers and high-pressure experimentalists who need mixture data or a workable proxy. A reader in that group would get value from the new numbers and the experimental angle even if they treat the precise thresholds as starting points rather than final answers. It deserves peer review because the claims are specific enough for referees to check the methods and ask for the missing benchmarks.

Referee Report

2 major / 1 minor

Summary. The manuscript uses density functional theory molecular dynamics (DFT-MD) simulations to examine the miscibility, molecular stability, and electrical conductivity of hydrogen-neon mixtures at pressures up to 10 Mbar and temperatures up to 10000 K. It reports that the minimum pressure for phase separation is substantially lower than in H-He mixtures, that neon stabilizes H2 molecules more effectively than helium, and that this leads to a reduction of several orders of magnitude in electrical conductivity relative to pure hydrogen. The work positions H-Ne mixtures as a useful experimental surrogate for phase separation studies due to neon's larger X-ray scattering cross section.

Significance. If the results are robust, they would provide valuable constraints on the interior structure and evolution of gas giants by clarifying how heavier elements influence hydrogen miscibility and metallization. The pronounced stabilization effect and conductivity suppression offer mechanistic insight into mixture behavior at planetary conditions, while the surrogate proposal could enable targeted X-ray experiments that are otherwise difficult with helium. These outcomes would strengthen links between ab initio simulations and observational models of Jupiter and Saturn.

major comments (2)

Methods section: The exchange-correlation functional employed in the DFT calculations is not reported. Different functionals are known to shift the molecular-to-atomic transition and metallization pressures by several Mbar; without this choice and any benchmarks against QMC or hybrid-functional data for the H-Ne system, the quantitative claims for the phase-separation threshold and conductivity drop at 10 Mbar / 10000 K cannot be assessed for systematic error.
Methods section: Simulation details including system size (number of atoms), equilibration times, production run lengths, and error estimation procedures for the reported phase-separation pressures and conductivity values are absent. These parameters are load-bearing for establishing the statistical reliability of the central results on lower miscibility pressure and orders-of-magnitude conductivity reduction.

minor comments (1)

Abstract: The phrase 'several orders of magnitude' reduction in conductivity is stated without a specific factor or reference to a figure/table; adding a quantitative range would improve precision.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their thorough review and valuable comments. We address each major comment below and plan to incorporate revisions to improve the clarity and completeness of the Methods section.

read point-by-point responses

Referee: Methods section: The exchange-correlation functional employed in the DFT calculations is not reported. Different functionals are known to shift the molecular-to-atomic transition and metallization pressures by several Mbar; without this choice and any benchmarks against QMC or hybrid-functional data for the H-Ne system, the quantitative claims for the phase-separation threshold and conductivity drop at 10 Mbar / 10000 K cannot be assessed for systematic error.

Authors: We agree with the referee that the exchange-correlation functional was not explicitly reported in the manuscript. We will revise the Methods section to include this information. Additionally, we will expand the discussion to address the choice of functional, its known limitations for hydrogen systems (such as shifts in transition pressures), and cite relevant benchmark studies for pure hydrogen. While specific QMC or hybrid-functional benchmarks for the H-Ne mixtures at these extreme conditions are not available and would require new calculations outside the current scope, we will highlight this as a potential source of systematic uncertainty in our quantitative results. revision: yes
Referee: Methods section: Simulation details including system size (number of atoms), equilibration times, production run lengths, and error estimation procedures for the reported phase-separation pressures and conductivity values are absent. These parameters are load-bearing for establishing the statistical reliability of the central results on lower miscibility pressure and orders-of-magnitude conductivity reduction.

Authors: We acknowledge that the specific simulation parameters were not detailed in the current version of the manuscript. In the revised manuscript, we will add comprehensive information on the system sizes, equilibration and production run durations, and the procedures used for estimating errors in the phase-separation pressures and electrical conductivity calculations. This will allow for a better assessment of the statistical robustness of our findings. revision: yes

Circularity Check

0 steps flagged

No circularity: results are direct DFT-MD simulation outputs

full rationale

The paper presents its central findings—the lower phase-separation pressure in H-Ne versus H-He, neon-induced stabilization of H2 at 10 Mbar and 10000 K, and the orders-of-magnitude conductivity drop—as direct numerical outputs from density-functional-theory molecular-dynamics trajectories. No analytical derivation chain, fitted-parameter prediction, or self-citation is invoked to obtain these quantities; they are computed quantities under the chosen XC functional and simulation protocol. The methodology section describes standard DFT-MD procedures without reducing any reported pressure or conductivity value to a quantity defined by the same study’s own fit. Self-citations, if present, concern only prior methodological validations and are not load-bearing for the mixture-specific claims. This is the normal, non-circular case for a first-principles simulation study.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The work rests on standard DFT-MD methodology whose accuracy at these conditions is an untested domain assumption; no new free parameters, ad-hoc axioms, or invented entities are introduced in the provided abstract.

axioms (1)

domain assumption Density functional theory with the chosen exchange-correlation functional sufficiently describes the electronic interactions and molecular bonding in dense H-Ne mixtures at 10 Mbar and 10000 K.
All reported phase behavior and conductivity results depend on this unverified approximation.

pith-pipeline@v0.9.0 · 5458 in / 1316 out tokens · 38484 ms · 2026-05-10T15:08:01.681407+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

3 extracted references · 3 canonical work pages

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