The accuracy of dynamical models for reactive scattering of molecular hydrogen, H 2 , from metal surfaces is relevant to the validation of first principles electronic structure methods for molecules interacting with metal surfaces. The ability to validate such methods is important to progress in modeling heterogeneous catalysis. Here, we study vibrational excitation of H 2 on Cu(111) using the Born-Oppenheimer static surface model. The potential energy surface (PES) used was validated previously by calculations that reproduced experimental data on reaction and rotationally inelastic scattering in this system with chemical accuracy to within errors ≤ 1 kcal∕mol ≈ 4.2 kJ∕mol [Díaz C, et al. (2009) Science 326:832-834]. Using the same PES and model, our dynamics calculations underestimate the contribution of vibrational excitation to previously measured time-of-flight spectra of H 2 scattered from Cu(111) by a factor 3. Given the accuracy of the PES for the experiments for which the Born-Oppenheimer static surface model is expected to hold, we argue that modeling the effect of the surface degrees of freedom will be necessary to describe vibrational excitation with similar high accuracy.Born-Oppenheimer approximation | dissociative chemisorption | molecule-surface scattering R eactions of molecules with metal surfaces are of tremendous importance, as the production of most man-made chemicals involves heterogeneous catalysis by a metal surface. One of the achievements recognized by the 2007 Nobel Prize in Chemistry, awarded to G. Ertl, was the detailed description of the sequence of elementary molecule-surface reactions by which ammonia is produced (1). In such reaction sequences, the dissociative chemisorption of molecules often plays a prominent role.In the present state of the art, theoretical calculations can reproduce overall rates of heterogeneously catalyzed reactions like ammonia production to within an order of magnitude (2). Achieving greater precision has been hampered by the lack of accuracy inherent in the density functional theory (DFT) used to calculate the interactions of molecules with metal surfaces.Because dissociative chemisorption involves stretching bonds until they rupture, reactivity of molecules on surfaces is closely related to transfer of energy to and from the vibrational degrees of freedom (3). Thus vibrationally inelastic scattering can be a sensitive probe of the barrier region of the potential energy surface (PES), and experiments (4, 5) and theory (6, 7) suggest that this process is governed by the same region of the PES as dissociative chemisorption: The picture of vibrational excitation occurring in competition with dissociative chemisorption is that it results from a stretching of the molecule as it approaches the transition state (4, 5). In principle, experiments on this competition may reveal much interesting information. For instance, comparison of data on vibrationally inelastic scattering and reaction on a surface has been suggested to provide information on whether these pro...