Potential energy surfaces are central features of atomistic models used to explain the properties of materials: they represent the potential fields, created by electrons, in which nuclei move. This assumption originates from the large difference in mass between nuclei and electrons and it is fundamental in nearly all theories and pictorial representations of atomic and molecular processes. The implications of this approach, which is based on an analysis by Born and Oppenheimer nearly 80 years ago in the early development of quantum mechanics1, are not often challenged. A consequence of the approximation is that only one electronic state needs to be considered as electrons are thought to adjust instantaneously to the movement of the nuclei. Therefore excited electronic states, or transitions between states, are often ignored when simulating processes such as chemical reactions and molecular dissociations in the gas phase. But will this approximation still be valid when it comes to scattering and dissociation reactions on metal surfaces? A study by Nieto et al. published in Science indicates that it does indeed hold2.

It has been known almost since the advent of quantum mechanics that metal surfaces have a continuous range of electronic states, giving metals their special properties such as electrical conductivity. A key question is whether chemical reactions on metal surfaces — such as the catalytic processes central to the production of many important chemicals — can be described by a single potential energy surface. It can be argued that the band of continuous electronic states in metals implies a breakdown of the Born–Oppenheimer approximation for such systems. Indeed, recent observations of photons and electrons produced following chemical reactions or collisions of some molecules with metal surfaces provide experimental support for this idea3,4.

The most exquisite way of probing a molecular event when the reactants are in selected quantum states is to monitor the change in trajectory of an incident beam of molecules following a collision process (scattering), through measurement of the diffraction angles. On the theoretical front it has been possible for several years to perform calculations of such processes for simple chemical reactions in the gas phase with, essentially, no approximations except that of Born and Oppenheimer5. Thus the potential energy surface for reactions such as H + H2 or F + H2 can be calculated using highly accurate ab initio quantum chemistry. Then it can be included in quantum dynamics calculations in all degrees of freedom to predict the angles at which the products are formed. Comparison of such theoretical predictions with experiments have confirmed the influence of quantum effects such as scattering resonances (that is, long-lived vibrational states) on the outcome of chemical reactions6. Furthermore, the perfect consistency between theory and experiment provides a strong case for the need to consider just one potential energy surface. In other words, the validity of the Born–Oppenheimer approximation is verified for these gas-phase reactions.

Now Nieto et al.2, by comparing theory and experiment, provide convincing evidence that the approximation also holds for the scattering and dissociation of H2 molecules into single hydrogen atoms on platinum surfaces. First, they use a single potential energy surface, obtained from a model that reliably describes the H2–Pt system, to calculate the probability of scattering at different angles. Comparison of these probabilities with the experimental distribution of angular scattering shows exceptional agreement. In addition, agreement is good for the theoretical and experimental probabilities for the dissociation reaction that accompanies chemisorption of H2 on platinum (illustrated in Fig. 1). This is strong evidence for the applicability of the theory. The comparison also suggests that an electronically adiabatic theory – a theory that considers electronically excited states as insignificant — is sufficient to predict and explain the experimental observables for this reaction.

Figure 1: Smooth separation.
figure 1

Dissociation of an H2 molecule on a platinum surface can be described accurately by a quantum mechanical treatment with a single potential energy surface.

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Examination of the potential energy surface provides an explanation as to why the electronically adiabatic theory is sufficient. The energy available to the reactants is not significantly enhanced as the chemisorption energy is relatively small. This seems to be a general feature for the interaction of H2 with metal surfaces. It is related to the fact that H2 cannot easily accept electrons from the metal, in contrast to molecules such as NO. Other calculations on the probability of electronic excitations in H2/metal systems confirm this view7.

The one slight weakness is that density functional theory has been used to calculate the potential energy surface. Comparison with highly accurate ab initio quantum chemistry calculations for simple gas phase reactions shows that density functional theory does not always predict reaction barriers accurately8. But the extensive comparisons that have now been done between theory and experiment for the H2/metal system do suggest that the density functionals used for this system are reliable.

The conclusions of this study are good news for the theoretical treatment of scattering and reactions of molecules such as H2 on metal surfaces. The use of the idea of the potential energy surface has held up well and the prospects for extending this type of rigorous quantum theory to other molecule–surface systems are promising. This might include dissociations of more complicated — and highly industrially relevant — molecules such as NH3 and CH4 on platinum and other metals.