The adsorption interaction between a water cage and a methane molecule, shown by the cage-methane PMF (potential of mean force), is key to understanding hydrate formation mechanisms. We investigate how the cage type and adsorption face affect the PMF. The PMF is found to depend on the face size rather than the cage type, and the adsorption interaction becomes stronger as the face size increases. However, once the face becomes 7-membered, it no longer adsorbs methane but allows methane crossing it to enter the cage. The results suggest that a preferential direction may exist during hydrate nucleation and growth. (C) 2013 Elsevier B. V. All rights reserved.