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

The properties of hydrogen at high pressure have wide implications in astrophysics and high-pressure physics. Its phase change in liquid is variously described as metallization, H-2 dissociation, density discontinuity, or plasma phase transition. It has been tacitly assumed that these phenomena coincide at a first-order liquid-liquid transition. In this work, the relevant pressure-temperature conditions are thoroughly explored with first-principles molecular dynamics. We show that there is a large dependency on the 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, H-2 dissociation becomes a smooth crossover in the supercritical region that can be modeled by a pseudotransition, where the H-2 <-> 2H transformation is localized and does not cause a density discontinuity at metallization. Thermodynamic anomalies and the counterintuitive transport behavior of protons are also discovered even far beyond the critical point, making this dissociative transition highly relevant to the interior dynamics of Jovian planets. Below the critical point, simulation also reveals a dynamic H-2 -> 2H chemical equilibrium with rapid interconversion, showing that H-2 and H are miscible. The predicted critical temperature lies well below the ionization temperature. Our calculations unequivocally demonstrate that there are three distinct regimes in the liquid-liquid transition of warm dense hydrogen: A first-order thermodynamic transition with density discontinuity and metallization in the subcritical region, a pseudotransition crossover in the supercritical region with metallization without density discontinuity, and finally a plasma transition characterized by the ionization process at very high temperatures. This feature and the induced anomalies originate in the dissociative transition nature that has a negative slope in the phase boundary, which is not unique to hydrogen but is a general characteristic shared by most dense molecular liquids. The revealed multifaceted nature of this dissociative transition could have an impact on the modeling of gas planets, as well as the design of H-rich compounds.