After taking the published structures for each enzyme included in the set, our first step was to reoptimize the geometries to ensure that a consistent level of theory was used. Any constraints that had been applied in the previous studies (see SI for details) were maintained in our optimizations, and for some structures in the Hc model, ORCA's fragment optimization feature was used to ensure that correct configurations were maintained. The original DMML structures were altered preoptimization to replace the carbon atoms at the QM/MM crossover points with hydrogens, so that they could be optimized as cluster models. For systems where multiple spin state surfaces were included, the structures were separately optimized in each spin state, and reaction energies and barrier heights were only calculated between structures of the same multiplicity to avoid any spin-crossover effects. Not all transition states of each reaction were able to be successfully reoptimized, so the final set contains no barrier heights for DMML, Co-NHase, PcrAB, Mo-Cu CODH, CDO and the singlet states of Hc and NiSOD.