To understand the unexpected change in the band gap, density functional theory (DFT) calculations have been performed for the reference systems Cs3Sb2Br9 and Cs3Bi2Br9 as well as the mixed Cs3(Sb0.5Bi0.5)2Br9. Ionic positions of the three systems were obtained by PBE+D3 geometry optimization, followed by refined electronic structure calculations using the hybrid HSE06 functional including spin-orbit coupling (see SI for computational details). The indirect bandgap and first direct transition of Cs3Bi2Br9 (Cs3Sb2Br9) were calculated to be 2.97 (2.91) and 3.10 (2.96) eV, respectively (see Table ). Previous GW-BSE calculations showed large exciton binding energies of ~300 meV, which are not captured in our DFT calculations. Correcting our DFT results by the large exciton binding energies results in a fair agreement with the experimental band gap values.