Warm dense hydrogen is studied in the region of fluid-fluid phase transition within the framework of the density functional theory. We report a procedure of obtaining metastable states and calculate the equation of state. Metastable states are diagnosed by pair correlation functions and values of conductivity. We obtain a strong overlapping through the density of metastable and equilibrium branches of pressure isotherms. This indicates the plasma nature of the phase transition. KEYWORDS density functional theory, equation of state, metastable states, warm dense hydrogen 1We present a method for the calculation of molecular metastable states of warm dense hydrogen within the framework of the DFT. The metastable states are diagnosed by PCFs and values of conductivity. The existence of metastable states indicates that the phase transition is a first-order phase transition. Note the following results:1 The metastable states are obtained for 700 and 1000 K isotherms. The isotherms have a peculiar sloped form with a strong overlapping of equilibrium and metastable branches and a rather small difference of specific volumes between branches. The overlapping found corresponds to the prediction [17] for the plasma phase transition. 2 The small jump of the specific volume at the phase transition prevents us from resolving metastable states at higher temperatures. 3 The phase coexistence line on the specific volume-pressure plane looks like a long and very narrow tongue. In this case, the accuracy of calculations does not permit us to locate the critical point within the interval of several thousand Kelvin. 4 The transition is overturned by a sequence of van der Waals loops, when an increase in temperature leads to a decrease in pressure, at which point the transition occurs as opposed to the conventional picture. 5 The findings from this study present arguments, in addition to, [5,8,49] in favour of the plasma nature of the fluid-fluid phase transition in warm dense hydrogen.

No abstract