Grand canonical Monte Carlo simulation results are presented for bulk sI methane hydrate. The description of hydrogen-bonded clathrate network allows the water molecules to move and rotate. A more idealized rigid structural model based on the van der Waals-Platteeuw (vdWP) theory is used for comparison. Occupancy isotherms and pressure versus temperature occupancy diagrams are computed for temperatures below 260 K and pressures up to 400 bar. It is found that the results obtained with the vdWP-like model are in qualitative agreement with experiment, though this model fails to account for structural transformations of water network in the vicinity of the melting point.