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Q9: DFT results highlight low adatom activation energy on (111) surfaces , but the model assumes idealized slab structures. How do defects (e.g., dislocations, grain boundary junctions) in real comp-Cu affect the predicted surface diffusion pathways? Are there experimental observations (e.g., TEM of bonding interfaces) that corroborate the DFT-predicted preferential migration on (111) planes?

We thank the reviewer for his careful reading and valuable consideration of our DFT results. We emphasize that DFT calculation carried out in this work serves as a qualitative evaluation of the influence from crystal orientation to the atomic diffusion at copper interfaces. Confining the crystal orientation as the exclusive variable allows us to approach the different void filling cases observed in coarse-grain copper, nt-Cu, and comp-Cu. On the other hand, the capability of evaluating the energy change of adding/missing an atom to some extent restrict the modeling of larger scale defects in bulk states through DFT, as mentioned by the reviewer. Therefore, we turned to simplify the atomic model to one-side surface model, to mimic grain boundaries clamping by interfaces on both-sides. Reference 43 and 44 in manuscript pointed out the crucial role of grain boundary diffusion from theoretical perspective, Reference 21 and 45 provided a molecular dynamics (MD) simulation aspect of atomic diffusion at the grain boundary. In particular, our DFT results are in good agreement with the MD results,

21 43 44 45