Based on the fact that CO * binds tightly, with a binding energy (< -1.8 eV on Pt(111)) and a diffusion barrier greater than k B T on Pt(111) (see SI), the application of HO approximation is reasonable. That said, the two lowest frequency modes, which corresponds to frustrated translation parallel to the surface (or surface diffusion), are a source of some anharmonicity. There are methods treating anharmonic modes more rigorously , but at present, those methods are designed for anharmonicity in the low-coverage limit. Incorporating coverage dependence and anharmonicity is beyond the scope of the present work, and we restrict our attention to the harmonic model, which is commonly used in the literature for CO • .