Intriguingly, this is but a fraction of the processes identified by APE. Analysis of the process lists reveals another group of 213 processes in which atoms access the top of the island, and a group of 360 processes that involve initial or final states that are clearly beyond the fcc-type structure of crystalline Pd(100). Such processes would ultimately lead to the nucleation of multi-layer islands, as well as restructuring, roughening and amorphization of the surface. These are highly interesting topics in their own right, and the APE framework establishes a means to systematically access the underlying atomic-level processes. Kinetically, such corresponding surface evolution would be penalized though, and will likely only kick in at elevated temperatures. Some of these intriguing offlattice or multi-layer processes are explicitly showcased in the SI. Much more intriguing is therefore a final group of 55 unique collective processes. These processes involve more atoms than the single-atom surface hopping or two-atom exchange diffusion mechanisms, yet all atoms in the initial and final states remain in fcc lattice positions. As illustrated in Fig. , they often correspond to sliding motions of entire rows of atoms. As also apparent from the example in the figure, many of these processes exhibit barriers in the same range as those of the classically considered diffusion mechanisms.