The main impediment to practical application is the toxicity of lead ions in halide perovskite absorbing materials. Computing tools based on density functional theory (DFT) were used to predict the intrinsic properties of potential for double perovskites to be effective and suitable for optoelectronic applications, replacing the conventional lead halide perovskites with environmentally friendly elements. The Generalized Gradient Approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) functional was used to screen homovalent alternatives for B and X-site ions in vacancy-ordered double perovskite Cs₂BX₆ (B=Pt, Ni, X= Cl, Br) for solar cell applications. Using the GGA with PBE functional, the band gap was calculated to be 1.411 eV, 0.482 eV, and 0.378 eV for the Cs₂PtBr₆, Cs₂NiCl₆, and Cs₂NiBr₆, respectively. The experimental band gap value of mother crystal's (Cs₂PtBr₆) was at 1.42 eV. Next, the DOS, PDOS and optical properties were computed using GGA with PBE functional. Then, the local density approximation (LDA) with Ceperley and Alder with Perdew and Zunger (CA-PZ) was executed to compare the GGA with PBE for electronic band structure. In addition, the OTFG ultra soft, OTFG norm conserving, ultra soft and norm conserving methods of pseudopotential were used for both GGA with PBE and LDA with CA-PZ to make and ensure the right or accurate DFT functional for those crystals. At last, the optical properties and their toxicity have been evaluated for their rational design of potential double perovskite materials with improved optoelectronic properties.