In contrast, the reduced effective masses of Cs 2 SnX 6 yield a weakly-bound exciton as expected, demonstrated by dominant band contributions at the Γ point to the first bright exciton state. Fig. also illustrates the differing trends in band structure for B = Sn/Ti, as X changes from I to Br to Cl, with a greater band gap increase and reduction in conduction band dispersion for Cs 2 SnX 6 than Cs 2 TiX 6 (such that the relative band gap energies of B = Sn/Ti changes from X = I (E g, Sn < E g, Ti ) to X = Cl (E g, Ti < E g, Sn )), due to the greater localization of the Ti d states. As discussed in Section S3, quasiparticle band gaps and thus exciton binding energies from GW (+BSE) remain overestimated for these vacancy-ordered compounds, consistent with recent studies which attribute this behavior to under-screening errors within the Random Phase Approximation (RPA) employed within GW . As such, to avoid this issue and obtain a reasonable estimate of the exciton binding energies in these systems, we also employed a constrained-supercell approach in which an exciton state is generated by controlling spin initialisation and band occupation. Here we calculate the exciton binding energy using hybrid DFT for multiple supercell sizes up to 972 atoms, then extrapolate to the dilute limit using the relevant scaling relationship to avoid supercell-size effects. With this approach, we obtain localised Frenkel exciton states as expected for each Cs 2 TiX 6 (Fig. , TOC), with extrapolated binding energies of 0.44 eV, 0.52 eV and 0.72 eV for X = I, Br, Cl (Fig. ), which when subtracted from the HSE06+SOC directallowed transition energies in Table brings the hybrid DFT optical transition energy into agreement with the experimental values in each case. For Cs 2 SnX 6 , the electron and hole remain delocalised across the supercell with this approach (under a maximum cell length of 23.1 Å; Fig. ), yielding extrapolated binding energies close to zero (Fig. ). Further details provided in Section S3.3.