  Water and hydrogen at high pressure make up a substantial fraction of the
interiors of giant planets. Using ab initio random structure search methods we
investigate the ground-state crystal structures of water, hydrogen, and
hydrogen-oxygen compounds. We find that, at pressures beyond 14 Mbar, excess
hydrogen is incorporated into the ice phase to form a novel structure with H4O
stoichiometry. We also predict two new ground state structures, P2_1/m and
I4/mmm, for post-C2/m water ice.
