To assess the size-dependent sintering behavior of Pt 1-8 /Al 2 O 3 , we prepared 8 different simulations as described in the Computational Methods section. The GMs and thermallyaccessible structures of the clusters at 750 K are shown in Fig. . Since our sintering code is based on Metropolis Monte Carlo, instead of kinetic Monte Carlo, we cannot directly access timescales from our simulations. However, we can analyze the final distribution of cluster sizes after some duration of the simulation, to see whether any cluster sizes persist, indicat- ing particular thermodynamic stability. Select final cluster size distribution histograms for different initial sizes are shown in Fig. , and the rest can be found in the Fig. . For initial cluster sizes Pt 2 (Fig. ), or Pt 4 (Fig. ), the final cluster size distribution is dominated by Pt 4 . This suggests that Pt 4 is a particularly stable cluster size; Pt 2 sinters to form primarily Pt 4 , and Pt 4 exhibits little sintering. This result is consistent with the experiment ISS results, which show that Pt 2 sinters significantly, whereas Pt 4 is essentially unaffected by 750 K heating. The final size distribution for Pt 7 initial size (Fig. ) is dominated by Pt 7 ; Pt 7 , like Pt 4 , is particularly resistant to thermal sintering. The exceptional stability of Pt 4 and Pt 7 can also be seen by examining the final size distributions for simulations starting with Pt 5 (Fig. ) or with a distribution of initial cluster sizes (Fig. ). In both cases, a variety of final sizes formed, however the distributions are dominated by Pt 4 and Pt 7 . Clearly, when either are present, either as the initial size or formed by sintering, they tend to persist. This persistence regardless of the starting point of the sintering simulation indicates that these two cluster sizes exhibit particular stability in the Ptn/Al 2 O 3 system. In contrast, for TiO 2 -supported Pt clusters, Pt 3 and Pt 7 were identified to be sinter-resistant, using the approach previously termed "competing pathways", which we now apply to this system.