| Summary: | CH<sub>n</sub> is the precursor unit for graphene synthesis. We have theoretically predicated a series of CH<sub>n</sub> structures with n = 1, 2, 4, 6, 8, 10, and 12 at elevated pressures (ambient pressure, 50, 100, 200, 300, 350, and 400 GPa) using evolutionary algorithms. The predicted CH and CH<sub>2</sub> structures are graphane-type and polyethylene over the whole considered pressure range, respectively. The molecular crystalline methane is predicted for the stoichiometry of CH<sub>4</sub>. The combination of methane and H<sub>2</sub> for CH<sub>6</sub>, CH<sub>8</sub>, CH<sub>10</sub>, and CH<sub>12</sub> up to 300 GPa are obtained. At 400 GPa, the mixture of polymer and H<sub>2</sub> for CH<sub>6</sub>, CH<sub>10</sub>, and CH<sub>12</sub> comes into play. From the computed enthalpy, higher pressure and more hydrogen concentration contributed to the decomposition (to carbon and H<sub>2</sub>) of CH<sub>n</sub> systems. The total density of states for these CH<sub>n</sub> structures show that only the CH<sub>12</sub> phase is metallic above 300 GPa. The rotational properties are traced in H<sub>2</sub> and the CH<sub>n</sub> structures. The CH<sub>4</sub> rotation is more sensitive to the pressure. The H<sub>2</sub> units are nearly freely rotational. Other structures of CH<sub>n</sub>, including fcc-type and experimentally known structures, are not competitive with the structures predicted by evolutionary algorithms under high pressure region. Our results suggest that the CH<sub>n</sub> (n > 4) system is a potential candidate for hydrogen storage where H<sub>2</sub> could be released by controlling the pressure.
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