Before discussing the PESs obtained with our approach, we note two caveats. Firstly, the ET dimension relies on DFT-based sampling that will introduce an uncertainty. We use local hybrid functionals to minimize the expected errors, which we expect to be small since the sampling remains close to the reactant minimum. Secondly, the PESs are expressed in terms of electronic energies and are not based on fully relaxed structures, which is particularly relevant for the product minimum, see SI for details. Therefore, a quantitative discussion of the energetic paths or of the PCET rate constant cannot be expected at the moment.