In order to assess the accuracy of our model prediction during optimization, particularly in the far OOD region, we performed explicit kMC simulations. Particularly for the largest particles, these simulations were extremely expensive, and we terminated many simulations early (after 20%-80% of the requested kMC steps had run) to reduce cost. Overall, the validating kMC simulations took >120,000 CPU-hours on AMD EPYC 7763 and Intel Xeon Gold 6330 CPUs, and individual simulations could take dozens of weeks. All optimizations using our trained hetero-GNN took ≈ 2,000 GPUhours on NVIDIA A100 GPUs. Because optimal particles often emerged early during the optimization process, this GPU-hour figure could probably be reduced by improving our optimization procedure. Coupled with the fact that the kMC simulations could not be directly used for gradient-based optimization, as they are not inherently differentiable, this indicates the massive acceleration in nanomaterial design that can be achieved using DL.