  By employing first-principles metadynamics simulations, we explore the 300 K
structures of solid hydrogen over the pressure range 150-300 GPa. At 200 GPa,
we find the ambient-pressure disordered hexagonal close-packed (hcp) phase
transited into an insulating partially ordered hcp phase (po-hcp), a mixture of
ordered graphene-like H2 layers and the other layers of weakly coupled,
disordered H2 molecules. Within this phase, hydrogen remains in paired states
with creation of shorter intra-molecular bonds, which are responsible for the
very high experimental Raman peak above 4000 cm-1. At 275 GPa, our simulations
predicted a transformation from po-hcp into the ordered molecular metallic Cmca
phase (4 molecules/cell) that was previously proposed to be stable only above
400 GPa. Gibbs free energy calculations at 300 K confirmed the energetic
stabilities of the po-hcp and metallic Cmca phases over all known structures at
220-242 GPa and >242 GPa, respectively. Our simulations highlighted the major
role played by temperature in tuning the phase stabilities and provided
theoretical support for claimed metallization of solid hydrogen below 300 GPa
at 300 K.
