Best-of-both-worlds computational approaches to difficult-to-model dissociation reactions on metal surfaces

Abstract

The accurate modeling of dissociative chemisorption of molecules on metal surfaces presents an exciting scientific challenge to theorists, and is practically relevant to modeling heterogeneously catalyzed reactive processes in computational catalysis. The first important scientific challenge in the field is that accurate barriers for dissociative chemisorption are not yet available from first principles methods. For systems that are not prone to charge transfer (for which the difference between the work function of the surface and the electron affinity of the molecule is larger than 7 eV) this problem can be circumvented: chemically accurate barrier heights can be extracted with a semi-empirical version of density functional theory (DFT). However, a second important challenge is posed by systems that are prone to (full or partial) electron transfer from the surface to the molecule. For these systems the Born–Oppenheimer approximation breaks down, and currently no method of established accuracy exists for modeling the resulting effect of non-adiabatic energy dissipation on the dissociative chemisorption reaction. Because two problems exist for this class of reactions, a semi-empirical approach to computing barrier heights, which would demand that computed and experimental dissociative chemisorption probabilities match, is unlikely to work. This Perspective presents a vision on how these two problems may be solved. We suggest an approach in which parameterized density functionals are used as in the previous semi-empirical approach to DFT, but in which the parameters are based on calculations with first principles electronic structure methods. We also suggest that the diffusion Monte-Carlo (DMC) and the random phase approximation (RPA) probably are the best two first principles electronic structure methods to pursue in the framework of the approach that we call first-principles based DFT (FPB-DFT) – providing DMC and the RPA with a steppingstone towards benchmarking and future applications in computational catalysis. Probably the FPB density functional is best based on screened hybrid exchange in combination with non-local van der Waals correlation. We also propose a new electronic friction method called scattering potential friction (SPF) that could combine the advantages and avoid the disadvantages of the two main existing electronic friction approaches for describing non-adiabatic effects: by extracting an electronic scattering potential from a DFT calculation for the full molecule–metal surface system, it might be possible to compute friction coefficients from scattering phase shifts in a computationally convenient and robust fashion. Combining the FPB-DFT and SPF methods may eventually result in barrier heights of chemical accuracy for the difficult-to-model class of systems that are prone to charge transfer. This should also enable the construction of a representative database of barrier heights for dissociative chemisorption on metal surfaces. Such a database would allow testing new density functionals, or, more generally, new electronic structure approaches on a class of reactions that is of huge importance to the chemical industry. Additionally, the difficult-to-model sub-class of systems we focus on is essential to sustainable chemistry and important for a sustainable future. Adding the database envisaged to large databases already existing but mostly addressing gas phase chemistry will enable testing density functionals that have a claim to universality, i.e., to be good for all chemical systems of importance. We also make a suggestion for how to develop such a generally applicable functional, which should have the correct asymptotic dependence of the exchange contribution to the energy in both the gas phase and the metal. Finally we suggest some improvements in the representation of potential energy surfaces and in dynamics methods that would help with the validation of the proposed methods.