The free energies of three reaction intermediates, HO 2 *, OH* and O*, were evaluated using quasi-stable structures at the interface between the catalyst surface and water modeled by hexagonal ice-like bilayers. The computed free energies were then used to calculate the rate constants of catalytic reactions. Since the overall reaction rate is balanced by the formation and removal rates of the intermediates according to the Sabatier principle, a volcano plot emerged when the overall rate was plotted against a property indicating the stability of intermediates, such as the redox potential of the OH* reduction reaction (5). The result indicates the existence of an optimal stability of intermediates for catalytic activity. This calculation result matched well with experimental results, showing that Pt is the optimal catalyst among pure metals. The model was also applied to Pt alloy catalysts and excellently reproduced the experimentally observed volcano plot for monolayer Pt skins on Pt alloy catalysts. The model also showed that ORR on pure Pt is limited by the removal of OH* [the reaction (5)] on the catalyst surface, and alloying accelerates this removal step by destabilizing OH*. Combining FP calculations with X-ray analyses also revealed the significant roles of the d-band structure and interatomic distance of surface Pt atoms. This knowledge paved the way for developing a series of core-shell catalysts, dealloyed catalysts, and shape-controlled alloyed nanoparticles. Furthermore, the FP-based model was applied to search for unknown active catalysts, successfully discovering new Pt alloys with early transition metal elements. It is noteworthy that this methodology has become a foundational method for predicting the activities of a wide variety of catalytic reactions, such as the hydrogen evolution reaction, oxygen evolution reaction, 32,33 CO 2 reduction reaction, N 2 reduction, and more.