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

Area of Science:

• Condensed matter physics
• Computational materials science
• Quantum chemistry

Background:

• Density functional calculations accurately model hydrogen phases but lack physical insight.
• Unusual features in the hydrogen phase diagram require further explanation.

Purpose of the Study:

• Develop a fast interatomic potential for molecular hydrogen.
• Provide physical insights into the observed phases and properties of hydrogen.

Main Methods:

• Development of a novel, fast interatomic potential for H2.
• Simulations reproducing orientationally disordered Phase I, broken-symmetry Phase II, and reentrant melt curve.

Main Results:

• The potential accurately reproduces key molecular hydrogen phases and the reentrant melt curve.
• High-pressure vibrational frequency drop is due to intermolecular coupling, not bond weakening.
• Liquid H2's higher density is attributed to a switch in favored molecular orientation (steric repulsion over quadrupole energy).
• Negative thermal expansion and hindered rotation in Phase I are explained by steric effects and frustration.

Conclusions:

• The new potential offers physical insights into hydrogen's complex phase behavior.
• Molecular orientation and steric repulsion are key factors governing high-pressure hydrogen properties.
• High-pressure Phase I of hydrogen is not a molecular rotor phase.