Halide perovskite ABX

_{3} has attracted increasing interest as a potential solar cell material because of its simple fabrication techniques and outstanding optoelectronic properties. ABX

_{3} perovskite materials have a high absorption coefficient, appropriate band gap (E

_{g}), and balanced electron and hole mobility [

1,

2,

3,

4,

5,

6]. In recent years, numerous researchers have focused on methylammonium lead trihalide perovskite (CH

_{3}NH

_{3}PbX

_{3}), metal halide perovskite (ABX

_{3}, A = Cs, Rb; B = Pb, Sn; X = Cl, Br, I), and CsPbI

_{3}, which have shown great potential [

7,

8].

Most of the researchers have studied the structural, electronic, and optical properties of CsPbX

_{3} (X = Cl, Br, I) using the density functional theory (DFT) and the WIEN2k package [

8,

9,

10,

11]. The E

_{g} tenability for CsPbX

_{3} was studied experimentally [

12], and the lattice modulation of Cs

_{1−x}R

_{x}PbBr

_{3} (R = Li, Na, K, Rb, x = 0–1) was also investigated [

13]. By doping the perovskite, the efficiency can be increased, as this can affect numerous electronic and optical properties [

7]. The structural and electronic properties of all of the inorganic mixed-halide perovskites, CsPb(Br

_{1−x}I

_{x})

_{3} and CsPb(Cl

_{1−x}Br

_{x})

_{3}, were investigated according to their halide composition using the Vienna ab initio simulation package (VASP) [

14]. The accuracy of DFT calculations, i.e., how close they are to experimentally measured values, has been a concern for DFT calculations on perovskite since their recent introduction into solar cell and LED applications [

15,

16,

17,

18,

19,

20,

21]. The accuracy of DFT calculations proved to be highly dependent on the exchange potential used in the calculations, such as local density approximation (LDA) [

22], Perdew–Burke–Ernzerhof generalized gradient approximation (PBE-GGA) [

23], modified Becke–Johnson GGA (mBJ-GGA) [

24,

25], Green function for the wave equation approximation (GW) [

26,

27], and hybrid functionals (HF) [

27,

28]. The HF and GW potentials have shown higher accuracy of the calculated band gap [

26,

27,

28], but these functionals were more computationally expensive than LDA or PBE-GGA. The calculated band gap using LDA or PBE-GGA potentials was strongly underestimated because these functions contain a self-interaction error [

29,

30]. For example, calculation of the E

_{g} has varied greatly among many recent DFT reports, occasionally with considerable deviations from experimental values (band gap reported between 1.359–1.75 eV [

8,

9,

31,

32] compared to experimental values of ~1.791 eV [

33,

34] for CsPbI

_{3} and 1.7–4.53 eV [

1,

8,

10,

35,

36,

37,

38] compared to experimental values of ~2.3 eV [

13,

39,

40,

41,

42,

43,

44] for CsPbBr

_{3}). However, very accurate measurements of the band gaps of semiconductors and insulators were obtained when an orbital-independent exchange-correlation potential, mBJ-GGA, was used. This depended solely on semilocal quantities and was competitive in accuracy with the expensive HF and GW methods [

24,

45]. The supercell calculations are usually performed to allow minor modification of the crystal structure by replacing one atom with another atom. The most successful approach, spectral weight (SW), which links the supercell band structure with the primitive basis representation, is based on a Bloch spectral density [

46]. One of the main challenges of supercell electronic structure calculations is to recover the Bloch character of electronic eigenstates [

46]. To our knowledge, there have been no studies so far for a spectral weight (SW) approach which can be used to unfold the band structure of inorganic perovskite compounds by fold2Bloch package [

46]. The fold2Bloch package was used in the past to unfold the band structure for other compounds such as GaAsBi [

47], group (III–V and II–VI) semiconductor solid solutions [

46], and graphene [

48].

In this study, a combination of CsPbI

_{3} and CsPbBr

_{3} was proposed to tune the electronic and optical properties, using the full-potential linear-augmented plane wave (FP-LAPW) method [

49,

50] within the framework of the DFT [

22], as implemented in the WIEN2K code [

51]. Here, an investigation into CsPb(I

_{1−x}Br

_{x})

_{3} (where x = 0.00, 0.25, 0.50, 0.75, 1.00) was performed to calculate the electronic and optical properties using PBE-GGA [

23] and mBJ-GGA [

24] methods. The structural properties were calculated using PBE-GGA potential. Unfolding the band structure of CsPb(I

_{1−x}Br

_{x})

_{3} compounds for a number of Br fractions was performed by calculating the Bloch SW, using the fold2Bloch package [

46] implemented in WIEN2k, in order to observe how the electronic properties of these compounds develop [

47]. The visualization for electronic and structural analysis (VESTA) program was used for atomic structure visualization [

52].