Structures of solid hydrogen at 300 K

We present simulated x-ray diffraction patterns (XRD) from molecular dynamics studies of phase transformations in hydrogen at room temperature. Phase changes can be easily identified in simulation, by directly imaging the atoms and measuring correlation functions. We show that the room-temperature XRD patterns for hydrogen phases I, III, IV, and V are very similar. The signatures of the transformations in XRD are weak peaks and superlattice reflections denoting symmetry breaking from the hexagonal-closed-packed (hcp) phase I, and a pronounced change in the c/a ratio. 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 molecules become increasingly confined to the basal plane and suggest the possibility of an unusual critical point terminating the phase I-III boundary line. With these results, we propose a paradigm shift, i. e. , that the predictions from density functional theory calculations should be seen as the most likely hypothesis. Specifically, we show that recent experimental results support the picture advanced by molecular dynamics simulations, and are inconsistent with the interpretation of an isostructural hcp transformation.