  High quality reference data from diffusion Monte Carlo calculations are
presented for bulk sI methane hydrate, a complex crystal exhibiting both
hydrogen-bond and dispersion dominated interactions. The performance of some
commonly used exchange-correlation functionals and all-atom point charge force
fields is evaluated. Our results show that none of the exchange-correlation
functionals tested are sufficient to describe both the energetics and the
structure of methane hydrate accurately, whilst the point charge force fields
perform badly in their description of the cohesive energy but fair well for the
dissociation energetics. By comparing to ice Ih, we show that a good prediction
of the volume and cohesive energies for the hydrate relies primarily on an
accurate description of the hydrogen bonded water framework, but that to
correctly predict stability of the hydrate with respect to dissociation to ice
Ih and methane gas, accuracy in the water-methane interaction is also required.
Our results highlight the difficulty that density functional theory faces in
describing both the hydrogen bonded water framework and the dispersion bound
methane.
