Structures of solid hydrogen at 300K

TL;DR

This study uses molecular dynamics to predict structural features of solid hydrogen at 300K, proposing a new paradigm for experimental validation by focusing on distinguishing candidate structures through measurable quantities.

Contribution

It introduces a paradigm shift encouraging experimentalists to test molecular dynamics predictions as the most probable hypotheses for hydrogen structures.

Findings

X-ray diffraction patterns for hydrogen phases are very similar at room temperature.

Molecular dynamics show atomic displacements significantly alter diffraction patterns from static structures.

Within Phase I, molecules become confined to the basal plane, indicating possible critical phenomena.

Abstract

We present results predicting experimentally measurable structural quantities from molecular dynamics studies of hydrogen. In doing this, we propose a paradigm shift for experimentalists -- that the predictions from such calculations should be seen as the most likely hypotheses. Specifically, the experimental results should be aiming to distinguish between the candidate low-energy structures, rather than aiming to solve the simplest structure consistent with the data. We show that the room temperature X-ray diffraction patterns for hydrogen phases I, III, IV and V are very similar, with only small peaks denoting symmetry-breaking from the hcp Phase I. Because they incorporate atomic displacements the XRD patterns implied by molecular dynamics calculations are very different from those arising from the static minimum enthalpy structures found by structure searching. Simulations also show that within Phase I the molecular becomes increasingly confined to the basal plane and suggest the possibility of an unusual critical point terminating the Phase I-III boundary line.