  Lead-free double perovskite materials (viz. Cs$_2$AgInCl$_6$) are being
explored as stable and non-toxic alternatives of lead halide perovskites. In
order to expand the optical response of Cs$_2$AgInCl$_6$ in visible region, we
report here the stability, electronic structure and optical properties of
Cs$_2$AgInCl$_6$ by sublattice mixing of various elements. Here, we have
employed %high-throughput screening using a hierarchical first-principles based
approach starting from density functional theory (DFT) with appropriate
exchange-correlation functionals to beyond DFT methods under the framework of
many body perturbation theory (viz. G$_0$W$_0$@HSE06). We have started with 32
primary set of combinations of metals M(I), M(II), M(III) and halogen X at
Ag/In and Cl site, respectively, where concentration of each set is varied to
build a database of nearly 140 combinations. The most suitable mixed
sublattices are identified to engineer the band gap of Cs$_2$AgInCl$_6$ to have
its application in optoelectronic devices under visible light.
