Understanding high pressure molecular hydrogen with a hierarchical machine-learned potential

Abstract

The hydrogen phase diagram has several unusual features which are well reproduced by density functional calculations. Unfortunately, these calculations do not provide good physical insights into why those features occur. Here, we present a fast interatomic potential, which reproduces the molecular hydrogen phases: orientationally disordered Phase I; broken-symmetry Phase II and reentrant melt curve. The H2 vibrational frequency drops at high pressure because of increased coupling between neighbouring molecules, not bond weakening. Liquid H2 is denser than coexisting close-packed solid at high pressure because the favored molecular orientation switches from quadrupole-energy-minimizing to steric-repulsion-minimizing. The latter allows molecules to get closer together, without the atoms getting closer, but cannot be achieved within in a close-packed layer due to frustration. A similar effect causes negative thermal expansion. At high pressure, rotation is hindered in Phase I, such that it cannot be regarded as a molecular rotor phase.

Taking materials dynamics to new extremes using machine learning interatomic potentials

“…In addition, they use physical model based descriptors that can describe up to 14 different structures in K [77,78] . Also, domain knowledge can be coupled with angle dependent structural descriptors, successfully capturing the change of chemical bond in solid H 2 [79] .…”

“…With the help of ML potentials, large-scale MD simulations become a powerful tool to understand small molecular crystal systems. The complex structural dynamics of H 2 [79,131] , H 2 O [75] , C 60 [133] and Azapentacene [132] have been investigated recently. Taking hydrogen as an example, we review the power of MLIP in investigating phase transformation in high-density molecular systems.…”

“…Taking hydrogen as an example, we review the power of MLIP in investigating phase transformation in high-density molecular systems. Zong et al [79] and Cheng et al [131] developed MLIPs for hydrogen separately almost at the same time. Cheng et al [131] reproduce both the reentrant melting behavior and the polymorphism of the solid phase.…”

“…They prove a continuous molecularto-atomic transition in the liquid, with no first-order transition observed above the melting line, which suggests a smooth transition between insulating and metallic layers in giant gas planets, and reconciles existing discrepancies between experiments as a manifestation of supercritical behavior. Zong et al [79] get an accurate potential to describe the complex Phase I, II, III and liquid phase of hydrogen. Phase I shows a typical hcp lattice of the molecular centers while the orientation of the H 2 molecular is disordered and remains rotating freely.…”

Two state model for critical points and the negative slope of the melting-curve

“…The solid phase in the region of negative slope is close packed, so the densification on melting cannot come from a collapsing open network. Curiously, hydrogen has a remarkably similar phase diagram to the alkalis which can be explained by competition between free rotors and quadrupole interactions [22][23][24] Density functional theory can reproduce the negative slope [25][26][27]. It also shows some anomalous behaviour in liquid heat capacity, compressibility, viscosity, and thermal expansion [28].…”