First-principles simulation of H-He Hugoniot and its comparison to experiments

The presence of an immiscible hydrogen–helium layer within Jupiter and Saturn has been proposed to explain both atmospheric helium depletion and Saturn’s excess luminosity. Recent laser-driven shock experiments on H₂–He mixtures revealed demixing along the principal Hugoniot under Jupiter-like conditions, with a sharp rise in reflectivity signaling phase separation. We perform ab initio molecular dynamics simulations based on finite-temperature density functional theory (MD-DFT) to investigate H–He mixtures along the Hugoniot. For the equation of state, we use the highly accurate thermal meta-GGA exchange-correlation functional Tr2SCANL. Optical properties, including reflectivity and conductivity, are computed using the recently developed range-separated thermal hybrid functional RS-KDT0, which offers superior band gap accuracy in warm dense matter. Tr2SCANL yields pressures in good agreement with experiment, while RS-KDT0 reflectivity predictions qualitatively match experimental data at low and high temperatures, outperforming standard PBE and global hybrid (KDT0) functionals. Moderate deviations from experiment appear at intermediate temperatures for RS-KDT0.