Gas hydrates are solutions of small guest solutes trapped
in the cages of a water lattice, β ice. Many gases form
hydrates under proper conditions of temperature and pressure,
at concentrations 2–3 orders of magnitude greater than
normal solvation for hydrophobic solutes. Methane hydrate
is among the most important. Its significance for the energy
sector is well appreciated, with estimates of methane held
ranging from tens to hundreds times that of conventional
reserves [1,2]. The threat to climate from the release of
methane greenhouse gas is of growing concern [3–6].
The phase equilibrium of β ice is central to hydrate
formation and stability. The Gibbs free-energy penalty for
liquid → β ice is small at 273 K [7], and the pressure can be
increased until occupation of the cages by guests results in
a thermodynamically favored hydrate.
Simulation studies are difficult because first-order phase
transitions proceed through a sequence of states with
coexisting phases, which are not easily sampled by conventional
methods.