Note that in all cases H prefers to bind to a Pt atom, where atop and bridge positions become competitive depending on the cluster size and composition. No thermally relevant isomer was found with H* adsorbed on either Ge or the support. In order to discriminate poor HOR catalyst candidates, we have set the optimal range for ΔGH* between -0.2 eV and 0.2 eV. At first glance, a remarkable size and composition dependence of ΔGH* can be observed, characteristic of the subnano regime. ΔGH* becomes more negative as the size increases, being the smallest Ptn clusters the ones with a ΔGH* closer to 0. Pt5 is the best candidate with ΔGH* = -0.02 eV, which outperforms commercial Pt/C (ΔGH* = -0.09 eV) 112 . Among the pure clusters, Pt8 has the strongest H* adsorption and thus, slowest Volmer step. Regarding the nanoalloys, a very small size or a high Ge content is required to yield low ΔGH* values. Indeed, the only clusters with ΔGH* values below the -0.2 eV threshold are Pt3Ge, Pt5Ge3 and Pt4Ge4 (ΔGH* = -0.08, -0.08 and -0.12, respectively). Nevertheless, despite ΔGH* being ~0.1 eV higher than that in Pt5, the PtGe nanoalloys may be equal or better than Pt5 when the HOR activity is weighted together with the CO tolerance. Generally, all clusters except for Pt4, present a negative ΔGH*, a behaviour that contrasts with the other 2D supports (vide infra). Moreover, a slight charge transfer from the cluster to H* is also observed, albeit no correlation with ΔGH* is found.