Electrocatalytic reactions on electrode surfaces are usually modeled with density functional theory calculations employing periodic boundary conditions. The proper description of charged metal surfaces requires large unit cells which precludes the application of fourth-rung density functionals or correlated wave function methods. Hence, the accuracy of the electronic structure calculations is difficult to assess and improve. We propose an alternative approach, where only a small part of the electrode is treated explicitly whereas the coupling to the semi-inifinite electrode is modeled through a self-energy term in the corresponding Green's function. The density matrix obtained from the integration of the Green's function features fractional occupations of the Kohn-Sham orbitals and the total (fractional) electron number changes over a reaction. The coupling to the electrode hence moves these calculations to the grand-canonical ensemble. This approach is closely related to the ab initio calculation of electron transport properties in molecular junctions. The effective reduction of the system size due to embedding then enables the use of density functionals with exact exchange and even correlated wave function methods. As a first approximation, we model the self-energy term by just two energy-independent parameters, namely an energy shift and a level broadening. To investigate the sensitivity of this embedding model to these two empirical parameters, we analyze the extent to which the model captures the properties of a 1-dimensional hydrogen ring when only a small number of unit cells is explicitly accounted for. Finally, we apply our approach to a realistic system and study the vibrational Stark effect of carbon monoxide chemisorbed to a silver surface under applied bias.

Toward electrocatalysis on metal clusters coupled to an electron reservoir

2019 Fall National Meeting

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