Structures of solid hydrogen at 300 K

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.