  Gas hydrates are systems of prime importance. In particular, hydrogen
hydrates are potential materials of icy satellites and comets, and may be used
for hydrogen storage. We explore the H2O-H2 system at pressures in the range
0-100 GPa with ab initio variable-composition evolutionary simulations.
According to our calculation and previous experiments, the H2O-H2 system
undergoes a series of transformations with pressure, and adopts the known
open-network clathrate structures (sII, C0), dense "filled ice" structures (C1,
C2) and two novel hydrate phases. One of these is based on the hexagonal ice
framework and has the same H2O:H2 ratio (2:1) as the C0 phase at low pressures
and similar enthalpy (we name this phase Ih-C0). The other newly predicted
hydrate phase has a 1:2 H2O:H2 ratio and structure based on cubic ice. This
phase (which we name C3) is predicted to be thermodynamically stable above 38
GPa when including van der Waals interactions and zero-point vibrational
energy, and explains previously mysterious experimental X-ray diffraction and
Raman measurements. This is the hydrogen-richest hydrate and this phase has a
remarkable gravimetric density (18 wt.%) of easily extractable hydrogen.
