  Understanding the high-pressure behavior of C-H system is of great importance
due to its key role in organic, bio-, petroleum and planetary chemistry. We
have performed a systematic investigation of the pressure-composition phase
diagram of the C-H system at pressures up to 400 GPa using evolutionary
structure prediction coupled with ab initio calculations and discovered that
only saturated hydrocarbons are thermodynamically stable. Several stable
methane-hydrogen co crystals are predicted: 2CH4 * H2, earlier obtained
experimentally, is predicted to have I4/m space group and 2-90 GPa stability
range at 0 K, and two new thermodynamically stable compounds 2CH4 * 7H2 (P-3m1
space group) and CH4 * 9H2 (Cm space group), as potential energy storage
materials. P21/c phase of methane is predicted to be stable at pressures < 8
GPa; bulk graphane (CH) was shown to be thermodynamically stable at 7-18 and
18-50 GPa and 0 K in the P-3m and Cmca phases, respectively; polyethylene is
shown to have a narrow field of stability. We report the p-T-x phase diagram of
the C-H system and p-T phase diagram of CH4.
