## Summary Battlemage GPUs support sub-group sizes 16 and 32, incompatible with NBNXM cluster size of 4 which is the default with DPC++. Making matters worse, our error message in this case recommends setting `GMX_GPU_NB_CLUSTER_SIZE=4`, which is the opposite of what needs to be done. For 2025: - Expand the docs - Improve the error message For 2026: - Consider changing the default value of `GMX_GPU_NB_CLUSTER_SIZE` to 8, which is incompatible with pre-Xe GPUs and inefficient for Alchemist, but is needed for Battlemage, PVC, and likely more future-proof; not to mention compatibility with non-Intel devices and consistency with the other GPU backends, incl. ACpp.