Supplementary Dataset Supporting the Main Article Entitled:Lone pair localization governs ferroelectric stability and excitonic properties in lead free halide perovskites

This study investigates the microscopic coupling between excitonic properties and ferroelectric stability in lead free halide perovskites using CsGeX3 (X = Cl, Br, I) as a model system. Using first principles density functional theory combined with GW and Bethe–Salpeter equation calculations, we establish a unified framework linking exciton binding energy, dielectric screening, and spontaneous polarization to the stereochemically active Ge 4s2 lone pair. We demonstrate that electronic asymmetry driven by lone pair localization governs ferroelectric strength across the halide series. Partial Rb substitution acts as chemical pressure, enhancing polarization while preserving visible light absorption and avoiding mid gap states. Hydrostatic strain provides an additional route to tune band gaps and excitonic binding. The results identify lone pair localization as a transferable descriptor connecting charge redistribution, dielectric response, and excitonic confinement, providing design principles for multifunctional lead free optoelectronic materials.