We further investigate the geometry of the VCl -on the Cs2AgBiCl6 surface upon inclusion of SOC corrections, as SOC could potentially destabilize the formation of metal dimers as known for the lead-halide perovskites. We optimize the structure containing the VCl -vacancy using PBE+SOC within the Quantum Espresso software package. Comparison of the PBE0 and PBE+SOC optimized structures suggest the stability of the Ag-Bi dimer, with bond lengths only slightly varying, see Figure . Thus, we can confidently trust the PBE0 structural properties despite neglecting SOC. We investigate the formation of self-trapped excitons (STEs) upon illumination. Figure summarizes the STE formation mechanism. Optimizing the system in the triplet state, using the PBE0 functional, results in the localization of the exciton, see Figure . The localization of the electron Kohn-Sham orbital is accompanied by an increase in Bi-Cl bond lengths from 2.7