Strategies to replace the divalent B-site cation in halide perovskites, while retaining the BX 6 octahedral motif, have led to the exploration of A 2 BB'X 6 double perovskites with a pair of monovalent and trivalent cations at the B and B' sites, as well as the A 3 B 2 X 9 'vacancy-ordered perovskites', where a trivalent B cation is combined with a 1/3 vacancy of the B-site to satisfy electroneutrality. Issues of indirect and/or large band gaps in these materials has led to the emergence of A 2 BX 6 vacancy-ordered double perovskites (VODPs), where now the combination of a tetravalent cation and a 50 % vacancy of the B site is employed, giving a checkerboard arrangement (Fig. ). Also known as defective or tetravalent perovskites, these compounds are actually one of the decomposition products of conventional ABX 3 perovskites, for example CsSnI 3 which breaks down to form Cs 2 SnI 6 . A 2 BX 6 vacancy-ordered perovskites have shown promise for optoelectronic applications, with good stability under air, moisture, light and thermal stresses, as well as being solutionprocessable and non-toxic. As with the single and double perovskites, the frontier orbitals of the B-cation and X-anion govern the conduction and valence band-edge properties, respectively. This combination allows tunability in the energy gap, absorption profile and carrier effective masses for example. The A-site species, on the other hand, is a large monovalent cation such as Cs + which behaves as a spectator, dictating the spacing between BX 6 octahedra but with no direct contribution to the band edges. A crucial difference to the ABX 3 perovskite template, is the lack of corner-sharing BX 6 octahedra caused by vacancy introduction. Consequently, the crystal structure is comprised of isolated octahedra and thus an effective zero-dimensional (0D) framework, with this dramatic reduction in connectivity being a key factor in the properties of this material family. Research efforts in this area initially focused on the Sn-based compounds (A 2 SnX 6 ), but have since expanded so that a range of tetravalent species have successfully been implemented in A 2 BX 6 materials, including Te, Pd, Zr and Pt. While some of these compounds have shown promise as potential white-light and tunable emitters, it is the Sn and Ti-based materials which have shown the most promising results in the context of solar photovoltaic applications, and thus received the majority of research attention. Cs 2 SnI 6 was originally used as a hole-transporting layer in dye-sensitized solar cells, for instance, achieving efficiencies of 8 %, while a Cs 2 TiBr 6 photovoltaic device demonstrated a modest efficiency of 3 %. The poor performance of these materials has been attributed to relatively weak visible light absorption and indirect band gaps. As issues of defect intolerance and operational instability becoming apparent for Cs 2 SnI 6 , there is growing interest in the Ti-based compounds. The effects on structure, stability and electronic properties in going from the Group 14 d 10 s 0 Sn 4+ to Group 4 d 0 s 0 Ti 4+ cations have been probed, however the performance limits of these materials remains an open question. Notably, while theoretical methods are found to successfully reproduce the experimental electronic structure of the Te-and Sn-based compounds, 11 a major discrepancy exists for the d 0 Ti-based compounds, with severe overestimation of the experimental band gap by both hybrid Density Functional Theory (DFT) and Green's function (GW ) methods. So extreme is the error, that these theoretical methods actually yield qualitatively incorrect relative band gap energies for the Sn vs Ti compounds, as we show in this study.