Mechanisms and Rates of InterstitialH2Diffusion in CrystallineC60

Parallel replica dynamics and minimum energy path calculations have been used to study the diffusion mechanisms of ${\mathrm{H}}_2$ in fcc ${\mathrm{C}}_{60}$. Isolated interstitial ${\mathrm{H}}_2$ 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 ${\mathrm{H}}_2$ molecule diffusing through an already occupied O site, creating an ${\mathrm{H}}_2$ 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 ${\mathrm{H}}_2$ in fcc ${\mathrm{C}}_{60}$.