While these sinter-resistant sizes are specific to the Pt n /Al 2 O 3 system, the general conclusions that (i) sintering of subnano cluster is a size-dependent phenomenon, (ii) there are "magic numbers" of sintering stability, (iii) sintering is driven by the entire ensemble of thermally-accessible isomers, with metastable states driving sintering and (iv) several spatial regimes of sintering (global, nucleation-like, mixed) exist which can be used as signatures of cluster sintering stability in experiment. These key findings should remain valid for many systems, as we have already shown the existence of sinter-resistance size dependence for a different support. Identifying cluster sizes that may persist under reactions conditions is essential, as it can be used to target them as potential active sites, either through direct synthesis, or by their formation during reaction. While systems under catalytic conditions will have bound adsorbates, we expect these general findings to hold, and will address the role of adsorbates on sintering in future work.