Semi-local DFT including dispersion corrections (PBE+D3) was also found to accurately reproduce the experimental lattice constants (Table ). The change in lattice parameter (∆a D3 ) is consistent within each halide subclass, irrespective of the B-site identity (Sn or Ti), reflecting the expected inter -(BX 6 -BX 6 ) rather than intra-octahedral (B-X) origin of these vdW interactions. Moreover, we demonstrate the importance of dispersion interactions between the BX 6 molecular blocks on the electronic properties, showing the calculated energy band gap to shift by 0.04 -0.31 eV in the optimized crystal structure. There is an increasing sensitivity of the band gap to the lattice parameter as we move down the halogen group (Cl → Br → I), as the through-space B-X and X-X interactions in the conduction and valence bands strengthen with larger X p orbitals, also explaining the reduced band gap shifts for B = Ti due to the more localized d orbitals. We further note a sensitivity of the electronic band gap on the DFT functional choice for geometry optimization, with a band gap 0.4 eV lower (-40 %) obtained for Cs 2 SnI 6 using semi-local DFT (PBEsol) for structure relaxation. Hybrid DFT including dispersion corrections was employed for all further DFT calculations in this study. The electronic band structures, density of states and charge densities at the valence band maximum (VBM) and conduction band minimum (CBM) for Cs 2 TiI 6 and Cs 2 SnI 6 are shown in Figs. and. While Cs 2 SnX 6 exhibit direct electronic band gaps at Γ, Cs 2 TiX 6 have indirect gaps with the CBM at the X high-symmetry k-point and the VBM remaining at Γ -in agreement with experimental studies. The direct/indirect gap energy difference (∆) is relatively small however, with ∆ = 0.06 eV, 0.07 eV and 0.04 eV for the I, Br and Cl isomorphs respectively calculated using HSE06+SOC. As previously noted, the reduced ionic radius of Ti 4+ vs Sn 4+ -resulting in a much lower energy difference between the t 1g ψ VBM and t 1u ψ VBM-1 for B = Ti vs Sn, with ∆E t 1g /t 1u = 0.02 eV/0.38 eV, 0.07 eV/0.30 eV and 0.07 eV/0.07 eV for B = Ti/Sn; X = I, Br, Cl (using HSE06+SOC).