Crystal structure of atomic hydrogen

We have performed quantum Monte Carlo calculations on atomic metallic hydrogen at zero temperature and constant volume (r₉ = 1.31) for protons rigidly fixed to lattice sites and for protons having zero point motion. By studying a variety of crystal structures including bcc, simple cubic, simple hexagonal, diamond, and β-Sn, we show that low-coordination structures, especially diamond, are energetically favored for a range of pressures around 3.0 Mbars. Our results show that it is essential to treat the electrons beyond the local density approximation and protons beyond the harmonic approximation to predict the correct ground state structure.