Mechanism of proton-coupled electron transfer described with QM/MM implementation of coupled-perturbed density-functional tight-binding.

Coupled-perturbed equations for degenerate orbitals were implemented for third order density-functional tight binding, which allowed the use of Mulliken charges as reaction coordinates. The method was applied to proton-coupled electron transfer (PCET) reactions in a model system and thoroughly tested for QM and QM/MM setups (i.e., coupled quantum and molecular mechanics). The performed enhanced sampling simulations were stable, and the obtained potentials of the mean force were able to address the thermodynamic and kinetic features of the reactions by showing the expected topography and energy barriers. Hence, this method has the potential to distinguish between concerted and sequential mechanisms and could next be applied to proton-coupled electron transfer reactions in more complex systems like proteins.