Title: Velocity autocorrelations across the molecular-atomic fluid transformation in hydrogen under pressure
Author(s):

	G. Ruocco (Center for Life Nano Science @Sapienza, Istituto Italiano di Tecnologia, 295 Viale Regina Elena, I-00161, Roma, Italy; Dipartimento di Fisica, Universita di Roma La Sapienza, I-00185, Roma, Italy),
	T. Bryk (Institute for Condensed Matter Physics of the National Academy of Sciences of Ukraine, 1 Svientsitskii St., 79011 Lviv, Ukraine; Lviv Polytechnic National University, 79013 Lviv, Ukraine),
	C. Pierleoni (Université Paris-Saclay, UVSQ, CNRS, CEA, Maison de la Simulation, F-91191, Gif-sur-Yvette, France; Department of Physical and Chemical Sciences, University of L'Aquila and CNISM UdR L'Aquila, Via Vetoio 10, I-67010 L'Aquila, Italy),
	A.P. Seitsonen (Département de Chimie, École Normale Supérieure, 24 rue Lhomond, F-75005 Paris, France)

Non-monotonous changes in velocity autocorrelations across the transformation from molecular to atomic fluid in hydrogen under pressure are studied by ab initio molecular dynamics simulations at the temperature 2500 K. We report diffusion coefficients in a wide range of densities from purely molecular fluid up to metallic atomic fluid phase. An analysis of contributions to the velocity autocorrelation functions from the motion of molecular centers-of-mass, rotational and intramolecular vibrational modes is performed, and a crossover in the vibrational density of intramolecular modes across the transition is discussed.

Key words: molecular-atomic fluid transition, hydrogen fluid, velocity autocorrelation functions, ab initio molecular dynamics

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