Onset of metallization and related transitions in solid hydrogen

The onset of metallization in highly compressed solid hydrogen and a corresponding discontinuity in molecular vibrations have been linked in the literature to band-gap closure in an ordered hcp structure. We performed extensive first-principles calculations on the energetics and the electronic properties of solid hydrogen, and explored in detail variations in molecular orientations, bond lengths, and the c/a ratio, within an hcp lattice. We find that the commonly assumed structure is energetically unfavorable, and that lower-energy structures have wider band gaps to much higher density. Implications for the onset of metallization are discussed.