Abstract
Diffusion of sorbate molecules within a crystal and on the crystal surface can be driven by lattice vibration or by a purely deterministic Hamiltonian mechanism involving coupling between translational degrees of freedom. We delineate these two distinct transport mechanisms for a wide class of sorbate–lattice pairs. When the sorbate–lattice interaction is important, a Langevin model with a spatially non-uniform thermal noise, corresponding to non-homogeneous lattice–sorbate interaction, from our earlier work can be used such that explicit MD simulation of the lattice thermal bath can be circumvented.