In the DOS calculations (Figure ), for the SrBi3O4Cl3 (x=1) end member, the top of the valence band is primarily of O 2p character, and the bottom of the conduction band is dominated by Bi 6p states. The energy gap between the valence and conduction band predicted by DFT is 1.60 eV, which is smaller than the experimentally observed indirect gap for SrBi3O4Cl3 of 2.58 eV. The PBE functional used in this work is well known to lead to an underestimate of experimentally observed band gaps but allows for a qualitative comparison with other compounds in the series. As Se is introduced into the structure to Sr0.75Bi3.25O4Se0.25Cl2.75 (x=0.75), the valence band becomes dominated by Se 4p states whereas there is little change to the character of the conduction band. The introduction of the Se 4p states results in a significant reduction in the PBE energy gap to 0.65 eV. This 0.95 eV drop in the band gap as Se is introduced into the structure is consistent with the experimental measurements, which demonstrate a drop from 2.58 eV at x=1 to 1.47 eV for x= 0.75, or a 1.11 eV drop. As the fraction of Se increases in the structure, the number of Se states at the valence band edge increases, but there is only a small change in the energy gap from x=0.75 to x=0, again consistent with the only modest change in band gap observed experimentally across this range. In experiment, the band gap will be modified slightly by disorder, as orderings other than the lowest energy one are accessible as demonstrated by a lack of superstructure ordering and Se occupation of both anion sites observed across the series. Overall, the computational and experimental results support the interpretation of a semi-isolated Se state being introduced in the band gap, where its DOS and bandwidth depend on the amount of Se introduced into the system. The separated Se orbital manifold in the x = 0.75 supports the interpretation of a likely isolated Se orbital manifold in the x= 0.95 and x= 0.99 compositions; however, there is some level of Se-O hybridization in the in-gap states that must be taken into account, as evidenced by the non-trivial O contribution to the in-gap Se states.