The question of whether hydrogen and water mixtures are miscible at planetary temperatures and pressures deep in the interior of planets like Uranus and Neptune remains unresolved. In 2021, Bergermann et al  used the Gibbs-ensemble Monte Carlo simulation method and found a huge miscibility gap which shows reasonable agreement with the experimental data of Bali et al . Soubiran and Militzer  conducted simulations based on density functional theory (DFT-MD) and predicted hydrogen-water mixtures to be completely miscible under those conditions. To resolve this problem, we performed extensive DFT-MD simulations for water-hydrogen mixtures at temperatures of 1000 K < T < 2000 K and pressures of 40 kbar < p < 300 kbar. By using the ideal entropy and calculating the free enthalpy we found hydrogen-water to be partially immiscible at temperatures of T = 1000 K and pressures of 40 kbar < p < 120 kbar. Despite the fact, that the non-ideal entropy is still neglected, our simulations provide evidence, that hydrogen-water mixtures might be immiscible under temperature and pressure conditions deep in the interior of planets like Uranus and Neptune. These results are important for interior and evolution models for ice giant planets because H2-H20 demixing would induce compositional gradients which could inhibit convection and, therefore, the cooling of those planets [4, 5].
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 F. Soubiran and B. Militzer, The Astrophysical Journal, 806 (2015)
 R. Helled, N. Nettelmann and T. Guillot, Space Sci. Rev., 216 (2020)
 E. Bailey and D. J. Stevenson, Planet. Sci. J., 64 (2020)