LAUSR

The temperature (T ) and density (ρ) conditions at which hydrogen undergoes a molecular-toatomic (MA) transition is crucial to our understanding of the gas-giant planets such as Jupiter and Saturn. First-principles (FP) calculations suggest that this transition is coincident with metallization and acts as a catalyst for hydrogen-helium demixing, which has significant consequences for models of planetary interiors. Prediction of this transition boundary has proven to be difficult using FP methods. In particular, detailed comparisons of finite temperature density functional theory (FT-DFT) calculations of the MA transition in both the high-T , low-ρ regime, where the transition is largely T -driven, and the low-T , high-ρ regime, where the transition is largely ρ-driven, suggest that the transition is very sensitive to the exchange-correlation (xc) functional used in the calculation. Here we present a detailed comparison of previous multiple-shock electrical conductivity measurements with FT-DFT calculations employing various xc functionals to probe a regime where both T and ρ play an important role in the transition. The measurement results are found to be inconsistent with the semi-local xc functional PBE, and are in much better agreement with the nonlocal xc functionals vdW-DF1 and vdW-DF2. Furthermore, we show that the inconsistency with PBE likely stems from pressure errors associated with the PBE xc functional, resulting in calculated pressures that are too low at these T and ρ conditions. Together with previous comparisons at high-T , low-ρ and low-T , high-ρ these results provide a consistent picture for the MA transition over a wide T and ρ range. This picture may also provide insight into differences in experimental observations of the metallization of liquid hydrogen and deuterium in the low-T regime.