Equation of state of dense hydrogen and phase transitions

The equation of state (EOS) of warm dense hydrogen is treated theoretically by a free energy approach. At low densities hydrogen is a molecular fluid which can be described by fluid variational theory. Dissociation of molecules is treated self-consistently and thermodynamic properties of the neutral fluid can be given via standard relations, see [1] for details. At high densities hydrogen is predicted to be in a metallic-like state. In the chemical picture the fluid and plasma states are mixed and the total free energy is minimal in chemical equilibrium. Therefore, the chemical potentials of the molecular system, of the atoms, and of the electrons and ions are equal and a coupled system of mass action laws is derived which gives the composition of partially ionized hydrogen plasma. In simplified calculations applied to hydrogen and deuterium the charged particles were treated as ideal plasma. The partition function of the atoms was supposed to be density-dependent using an expression of Kerley [2] which leads to full ionization at high densities. Comparison of Hugoniot curves with shock wave experiments and other theories is performed in Fig. 1.