We performed global optimization for Pt7, Pt7Ge2, and Pt7Ge2C2 on α-alumina and then based all other calculations on the structures for the global minimum (GM) and two low-lying local minima (LM1 and LM2) isomers. Their top and side views are shown in Fig. , along with the energies relative to the GM, the spin states, 700 K Boltzmann populations (P700K), and the net charge on the clusters (ΔQ). The populations were calculated at 700 K to include isomers that would be populated during TPD cycling, where desorption is essentially complete at this temperature. For Pt7, the GM adopts a prismatic structure, which comprises 70% of the 700 K Boltzmann population. About 20 isomers were found within 0.5 eV of the GM, thus, due to the strong fluxionality, several Pt7 isomers are expected to be populated in the experimental temperature range. In contrast, only 6 and 5 isomers, respectively, were found within 0.5 eV of the GM for Pt7Ge2 and Pt7Ge2C2, making them unusually non-fluxional for a Ptbased cluster of this size. Given the DFT energies, the GMs of both Pt7Ge2 and Pt7Ge2C2 would dominate the distribution (P700K > 98%), with small LM1 and insignificant LM2 populations. Indeed, similarly low fluxionality was also noticed for Pt4Ge and Pt4GeC2. The isomers of Pt7Ge2 resemble the GM of Pt7 with all the Pt atoms exposed, and the Ge atoms either within or beneath the surface layer.