An approximation for the self-part of the van Hove function is derived. The self-part of the van Hove function is calculated by MD simulation for the diffusion of methane in ZK4 and silicalite and compared with the approximation. Fourier transform in space of the van Hove function yields the intermediate scattering function, the decay of which is used to determine the self-diffusion coefficient. Fourier transform in time yields the dynamic structure factor which can be compared with quasi-elastic neutron scattering results.