Metastable molecular fluid hydrogen at high pressures

Abstract

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.

Nonadiabatic effects and excitonlike states during the insulator-to-metal transition in warm dense hydrogen

“…Therefore the transition between two molecular phases can be envisaged: between the fluid of molecules in the ground state and the growing fluid clusters of excited molecules. This that can qualitatively explain results of 49 . Thus the concept of the plasma phase transition [51][52][53] should be probably reserved for the transition at ultrahigh temperature as has been suggested recently after careful equilibrium analysis of this IMT 18 .…”

“…Recently, the intriguing results have been published that reveal metastable states in fluid H 2 49 . The nature of these metastable states is not clear since they were found using the equilibrium FT-DFT approach.…”

Exciton Nature of Plasma Phase Transition in Warm Dense Fluid Hydrogen: ROKS Simulation**

“…[23,37,38] Later, the insulator-to-metal transition concept has become widespread. [39] The most common approaches are the first principles molecular dynamics (FPMD) based on density functional theory (DFT) [40][41][42][43][44][45][46][47][48][49][50] and quantum Monte Carlo methods. [51][52][53][54][55][56][57][58][59][60] At the same time, the plethora of methods cannot fully explain the key properties of the transition, such as a large isotope effect, [21] a large latent heat of transition, [61] as well as the observed differences in the moments of the detected increase in absorption and reflectivity.…”

“…This DFT model contrasts with the finite temperature (FT) DFT method that is used routinely for warm dense fluid H 2 calculations. [42,[44][45][46][47][48][49] Our previous study of the non-adiabatic dynamics of individual electronic excitations [62] showed the possibility of the appearance of localized excitons during the plasma phase transition (excitons in liquids are not unusual, e. g. these states were discovered in liquid Xe). [73,74] The lifetimes of such excitons in fluid H 2 turned out to be of the order of 10 fs, which is much larger than a typical time step in FPMD.…”

“…For this purpose, we deploy the restricted open‐shell Kohn–Sham (ROKS) method [64–72] within DFT that describes a single localized electronic excitation formed in fluid H 2 after the corresponding non‐adiabatic thermal excitation. This DFT model contrasts with the finite temperature (FT) DFT method that is used routinely for warm dense fluid H 2 calculations [42,44–49] …”