Even the most simple electrochemical reactions at a liquid / solid interface, of which proton transfer is of particular importance, still pose a great challenge to theory and simulation.  Modeling the transfer of protons and their discharge at electrified electrode surfaces is particularly challenging due to the fact that the solvent water cannot even approximately be decoupled from the elementary steps of proton motion. Rather the solvent participates in the reaction sequence. 

Different approaches based on electronic structure theory can be applied in principle to study the reaction of an individual proton at the electrolyte / metal interface. In particular the quantum-mechanical density functional theory (DFT) has been employed in many groups. Adequate statistic averaging over the multitude of possible proton pathways demands the use of approximate methods. In recent years the empirical valence bond (EVB) framework has been extended to study proton transfer reactions on metal surfaces. In this project we plan to extend the general EVB idea by developing a mixed quantum/classical framework for the proton discharge reaction, in which (all or some of) the elements of the EVB Hamiltonian matrix are calculated from quantum mechanics. This approach reduces or even completely saves the effort in fitting an entire reactive force field to the quantum chemistry data and can be used to systematically improve the description of the individual building blocks of the EVB Hamiltonian (i.e. its diagonal and its off-diagonal matrix elements).

It allows the investigation of the relative benefits of a whole class of theoretical descriptions of the relevant interacting subsystems such as reactive force fields (previous studies), tight-binding Hamiltonians, semi-empirical quantum mechanics, DFT cluster and periodic slab calculations up to a full ab initio treatment of (naked or embedded) clusters containing a proton-containing aqueous subcluster and a segment of the metal electrode. Furthermore, the relative importance of hydrogen bonding interactions versus dispersion and chemical bonding interactions will be investigated for the specific situations occurring near an electrochemical interface.