The selection of the cationic chemical space in the present work excludes the 3d transition metals, with the exception of Ti, which occurs mostly as Ti 4+ in a d 0 configuration. On one hand, this selection allowed us to limit the scope of this initial work for d 2 GNN to non-magnetic DFT calculations. On the other hand, this selection also addresses the hypothesis that high-performance materials for TCH might be found outside the realm of 3d transition metal oxides, where the vast majority of recent research efforts has been focused. While numerous discoveries of 3d oxides have been made over the past decade, especially including manganates, the general trend remains that these oxides split water only under dilute H 2 :H 2 O conditions. This drawback can be attributed to an insufficient reduction entropy, resulting from the presence of repulsive defect interactions that reduce the (atomic) configurational entropy contribution and from the lack of electronic entropy contributions. The strong electron correlations in the 3d shell cause localized electronic states, which nevertheless experience significant interactions with the ligands, as well as the existence of multiple oxidation states. As a consequence, the excess electrons introduced by the O vacancy defects tend to be strongly bound to the defect site, resulting in a large ionization energy, which precludes the electronic entropy contribution. While there could be exceptions to this trend for suitable combinations of the transition metal electron configuration and the ligand coordination environment, 57 a potentially rewarding alternative materials search strategy could focus on cations with more delocalized non-magnetic electronic states, as included in the present work, where a smaller ionization energy could enable water splitting under much more concentrated H 2 :H 2 O mixtures, which is currently achieved only in CeO 2 . While the present work does not directly address the ionization energy and resulting electronic en-tropy, it does provide the basis for selecting candidate materials for additional electronic structure studies with beyond-DFT methods that are required to address this question.