For systems containing both water and small ( 9Å) hydrocarbons, hydrates are an important part of the phase diagram. More information about the impact of hydrate formation can be found beginning at Hydrates. On a molecular scale, hydrates form when single, small guest molecules are encaged (enclathrated) by hydrogen-bonded water cages, which then combine as solid unit crystals in these nonstoichiometric hydrates. Hydrocarbon guest repulsions prop open different sizes of water cages, which combine to form the three well-defined unit crystal structures shown in Figure 1. These three common hydrate structures each have large and small cavities.