Crystal structure of atomic hydrogen

We have performed quantum Monte Carlo calculations on atomic metallic hydrogen at zero temperature and constant volume (rₒ=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 -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.