| Summary: | The structural, electronic, and optical properties of inorganic CsPb(I<sub>1−x</sub>Br<sub>x</sub>)<sub>3</sub> compounds were investigated using the full-potential linear augmented-plane wave (FP-LAPW) scheme with a generalized gradient approximation (GGA). Perdew–Burke–Ernzerhof generalized gradient approximation (PBE-GGA) and modified Becke–Johnson GGA (mBJ-GGA) potentials were used to study the electronic and optical properties. The band gaps calculated using the mBJ-GGA method gave the best agreement with experimentally reported values. CsPb(I<sub>1−x</sub>Br<sub>x</sub>)<sub>3</sub> compounds were wide and direct band gap semiconductors, with a band gap located at the M point. The spectral weight (SW) approach was used to unfold the band structure. By substituting iodide with bromide, an increase in the band gap energy (E<sub>g</sub>) values of 0.30 and 0.55 eV, using PBE-GGA and mBJ-GGA potentials, respectively, was observed, whereas the optical property parameters, which were also investigated, demonstrated the reverse effect. The high absorption spectra in the ultraviolet−visible energy range demonstrated that CsPb(I<sub>1−x</sub>Br<sub>x</sub>)<sub>3</sub> perovskite could be used in optical and optoelectronic devices by partly replacing iodide with bromide.
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