Rate Theory for Correlated Processes: Double Jumps in Adatom Diffusion

We study the rate of activated motion over multiple barriers, in particular the correlated double jump of an adatom diffusing on a missing-row reconstructed platinum (110) surface. We develop a transition path theory, showing that the activation energy is given by the minimum-energy trajectory which succeeds in the double jump. We explicitly calculate this trajectory within an effective-medium molecular dynamics simulation. A cusp in the acceptance region leads to a $\sqrt{T}$ prefactor for the activated rate of double jumps. Theory and numerical results agree.