  We present a study of the adsorption and diffusion of CH4, CO2 and H2
molecules in clathrate hydrates using ab initio van der Waals density
functional formalism [Dion et al. Phys. Rev. Lett. 92, 246401 (2004)]. We find
that the adsorption energy is dominated by van der Waals interactions and that,
without them, gas hydrates would not be stable. We calculate the maximum
adsorption capacity as well as the maximum hydrocarbon size that can be
adsorbed.The relaxation of the host lattice is essential for a good description
of the diffusion activation energies, which are estimated to be of the order of
0.2, 0.4, and 1.0 eV for H2, CO2, and CH4, respectively.
