Mechanisms and rates of interstitial H2 diffusion in crystalline C60

Parallel replica dynamics and minimum energy path calculations have been used to study the diffusion mechanisms of H2 in fcc C60. Isolated interstitial H2 molecules bind preferentially in the lattice octahedral (O) sites and diffuse by hopping between O and tetrahedral sites. The simulations reveal an unexpected mechanism involving an H2 molecule diffusing through an already occupied O site, creating an H2 dimer, with a lower activation barrier than diffusion into an empty O site. Kinetic Monte Carlo simulations of a lattice model based on these mechanisms indicate that events involving dimers greatly enhance the self-diffusion rates of interstitial H2 in fcc C60.