Structural and Electronic Properties of CBO and CFO the GGA+U approach, the CBO's lattice parameter c = 6.205 Å is overestimating the experimental value of 5.807 Å. This overestimation is well known as the PBE-functional tends to underbind the atoms in the unit cell. Additionally, the Hubbard U parameter can increase the lattice parameters as well. Nevertheless, the lattice parameters of CFO agree with the equilibrium lattice constants obtained experimentally and theoretically at the PBE+U level of theory. The photocatalyst's optical band gap should fall within the visible light spectrum to fulfill the essential requirement for an effective conversion of solar energy. to Cu-3d transition. However, the efficiency of d -d transitions is known to be lower in converting photons to photocurrent compared to O 2 -ligand to metal cation charge transfer transitions. Higher energy values are necessary to generate a significant photocurrent efficiency, leading to an optical absorption corresponding to the O-2p to Cu-3d transition. Moreover, the [CuO 4 ] 6 -units are isolated from each other in the bulk with a minimal distance of 3.10 Å between the Cu atoms in two neighboring [CuO 4 ] 6 -units. As a consequence, the photogenerated electrons and holes are localized in the [CuO 4 ] 6 -units, giving rise to a deceleration of the charge carriers and the formation of polarons. The polaron formation can be seen in the PDOS, as the states in the conduction band minimum are almost segregated from the states larger than 2 eV in the conduction band. The small polaron hopping is widely discussed in the literature as the main reason for the poor charge carrier transport in CBO. Introducing the U parameter to Cu and Bi gives a larger band gap of 1.91 eV, now laying within the range of 1.5 -2.11 eV as reported in experimental and theoretical studies. Literature has shown that applying the Hubbard parameter to the Cu-3d and Bi-5d states enhances the Coulomb interaction of both Cu-3d and Bi-5d states, thus increasing the band gap to a value in agreement with experiments. As indicated in Tab. 1, experimental studies can yield a lower band gap values of 1.5 -1.8 eV which could be a consequence of Cu defects in the material. Additionally, the nature of the magnetization of CBO is antiferromagnetic with an absolute magnitude of the magnetic moment of 0.68 µ B for the Cu atoms, which is in agreement with neutron diffraction measurements and DFT+U calculations. We will not elaborate any further on magnetization for both materials, as there are already a lot of publications cov-ering this topic. For CFO the valence band mainly consists of Cu-3d states at -3.5 eV and hybridized Fe-3d states at -8 eV to -6 eV. These hybridized Fe states occur only in the DOS calculated on the DFT+U level. In the DOS calculated without the U parameter, the Fe-3d states are distributed over the valence band with states at -5 eV and from -1 to 0 eV. The Fe states are thus more separated when using the DFT+U approach. In both methods, the O-2p