Atomic relaxations were performed with a total energy change threshold of 0.01 meV/atom and Hellmann-Feynman forces below 10 meV/Å. The atomic coordinates, including lattice constants of the polymorphs, were determined by minimizing the total energy E and the atomic forces. The generation of images for supercell and structure visualizations was performed using the VESTA software, and visual representations of the data were generated using the Matplotlib library in Python scripts. The band gaps and electronic interband transition energies predicted by DFT are significantly underestimated. Many-body calculations, based on DFT results, have been carried out for sesquioxides up to about 10 atoms. Despite the inclusion of 10 d-electrons per cation, polymorphs with 40 atoms or more in the unit cell require too much computational resources in GW calculations. Therefore, in this work, we apply a recently developed fast and accurate QP approach, the DFT+A-1 /2 method, which improves upon the conventional DFT-1 /2 scheme with excellent results for semiconductors, including correct adaptation of the binding energy of shallow core d-electrons.