Changes in properties of H₂ molecules at the fluid–fluid phase transition in warm dense hydrogen

Abstract

The warm dense hydrogen is studied in the region of the fluid–fluid phase transition in the density functional theory framework. A formalized definition of the term “molecule” based on geometry is proposed for calculating the concentration of molecules H2 and their lifetime. Correlations between density, conductivity, H2 molecules' lifetime, interatomic distance, and their dissociation degree are studied. The transition from molecular fluid to atomic fluid is not abrupt, while there is a drastic change in the H2 molecules' lifetime at the density, where the phase transition occurs. It can be interpreted as the ionization of H2 to H2+$$ {H}_2^{+} $$ plus e− at the phase transition, and the start of dissociation of H2+$$ {H}_2^{+} $$ to H plus H+ right after the phase transition.