Alignment to HER and OER: In addition to having an optical gap in the visible range, the electron removal and addition energies should be aligned to the redox potentials of the desired reactions. This alignment ensures that the process can happen thermodynamically. Therefore, we chose building blocks that are known to properly align with the case study of oxygen (OER) and hydrogen (HER) evolution reactions. At pH 0, the redox potentials of HER and OER are, respectively, -4.4 eV and -5.63 eV w.r.t. vacuum. For example, we have Ti clusters that individually tend to align well with HER. MOFs are a good platform to tune the band gap of Ti-based clusters using building block selection, 37,38 which we explore in this work. For OER, as an example, we selected an artificial cluster (mn39, with Mn(III)/Mn(IV)) mimicking the OER center in chlorophyll. This cluster is associated with the pivotal 4-electron transfer step responsible for generating O 2 during photosynthesis. Charge separation and charge-transfer character: Another aspect to look at is the choice of building blocks that could promote more efficient charge separation. This is important to reduce the possibility of charge recombination after excitation. In this regard, the nature of MOFs is particularly interesting because they can display metal node-to-linker or linker-to-metal node charge transfer. A necessary condition for the latter is the presence of low-lying metal states in the band structure. We chose Mn, Co, and Fe-based clusters with an open shell character that could contribute to a charge-transfer mechanism. Moreover, we designed MOFs with one-dimensional (1D) SBUs (rod-shaped, or rod MOFs) that can contribute to the effective separation of excited electron and hole. Rod MOFs are highlighted in Table and represented in Figures and.