In the calculations of the optical and loss spectra shown in Figs. 10 to 15, many-body effects, including excitonic contributions, were not considered. Instead, only QP effects were approximated. Fully accounting for these effects ab initio is challenging due to the large unit cells characteristic of many group-III sesquioxides. However, a rough estimation of excitonic effects can be made by examining the binding energy E b of the lowest-energy bound exciton relative to the QP absorption edge, along with its excitonic Bohr radius r B , using the Wannier-Mott approach. The anisotropy of the effective masses (see Table ) and the static electronic dielectric constants (see Table ) further complicates the estimation of binding energies. Consequently, we use averaged values for m * from both DFT+A-1 /2 and DFT, as well as for ε ∞ from DFT, as listed in Table . Additionally, we neglect the influence of hole masses on the interband mass due to the weak dispersion of the uppermost valence bands. Assuming that the mass of the hole in the electron-hole pair forming the exciton bound state is large compared to the electron mass m * , the exciton's internal motion can be described by the motion of the electron around the hole. This hydrogenic Wannier-Mott model yields an exciton binding energy of E b = R H m * /(mε 2 ∞ ), where R H = 13.605 eV is the hydrogen Rydberg, and an excitonic Bohr radius of r B = a B ε ∞ /m * . The binding energies (E b ) and exciton radii (r B ) derived using the isotropic Wannier-Mott model are provided in Table . The values of E b vary depending not only on the compound-whether Ga 2 O 3 or In 2 O 3 -but also on the specific polymorph. For Ga 2 O 3 , we observe binding energies ranging from 293 to 388 meV when using the QP mass, and from 167 to 221 meV when using the DFT mass. In contrast, for In 2 O3, the binding energies are lower, mental data. Our primary focus was on the electronic structures of the ten polymorphs, with QP excitation effects treated using the DFT+A-1 /2 scheme. The resulting band structures reveal weakly dispersive uppermost valence bands, while the lowest conduction band shows a distinct minimum with varying dispersion around the BZ center, largely independent of the polymorph. In general, the band gap value obtained using DFT+A-1 /2 method show excellent agreement with experimental data, not only in absolute values but also in general trends. This method successfully predicts, for instance, the largest band gap for the α phase of Ga 2 O 3 compared to all other phases. The lowest fundamental gap in all polymorphs (except one) is formed by a conduction band minimum near the Γ point and a valence band maximum located away from Γ. However, these indirect gaps are closely followed by direct interband transitions at Γ with only a slight energy increase. While the average band gaps increase significantly from In 2 O 3 to Ga 2 O 3 , the variation in gaps among the polymorphs is relatively small. Optical spectra, particularly the absorption coefficient onsets, mirror this band gap behavior. Additionally, polymorphs with lower crystal symmetry exhibit clear anisotropy in response to light polarization, which also affects the low-frequency behavior of the real parts of the dielectric tensor and, consequently, the static electronic dielectric constants. This anisotropy, in turn, influences the Coulomb interaction strength between optically excited electron-hole pairs, and together with the effective electron masses, determines the binding energy of band-edge excitons.