These metalloporphenes may be compared to radical cations and anions of annulenes, which tend to have low BLA. This leads to a small SOMO-SUMO gap, consistent with halfmetallic behavior in groups 1 and -1. Group 2. Adding two electrons to the porphene π-system (relative to group 0) completely fills up both rotationally related bands (Figure ), which leads to a D4h geometry and a relatively large band gap (>2 eV, Table ) in 1 Ti(IV) 4+ P 4-(Figure ). Its electronic structure is similar to that of benzene (Figure ), indicating that a general form of Huckel's rule (adding or removing two electrons reverses aromaticity) is applicable to the electronic structure of metalloporphenes. Group 2 metalloporphenes seem to be relatively rare, probably because they require metals with very high-energy d-electrons (i.e. strong reducing agents). Outlook. At this point, it may be useful to identify several potential areas of application for metalloporphenes: (i) Molecular wires. Due to their high conductivities, group 0 tt and 0 t metalloporphenes may be useful as molecular wires. Possible diamagnetic candidates are 1 Ti(IV)Cl2P and 1 Ni(II)P, both of which belong to 0 t . Preparing these metalloporphenes would also provide a simple test of the predictions made here. (ii) Tunable semiconductors. Semiconductive metalloporphenes (0 U , 0 UU , and 2) have a large variety of band gaps (1-3 eV), making them similar to quantum dots, 47 except that the electronic properties are controlled by choice of metal instead of by size. Accurate band gap measurements (e.g. for 1 Zn(II)P) will also enable the validation and refinement of these computational predictions. (iii) Mechanical properties. In 0 t and 0 U metalloporphenes, there exist two equivalent D2h orientations of each unit cell. As the barrier for their interconversion is low (~0.1 eV/unit cell, cf. Table , 3, and SI), the macroscopic properties of these metalloporphenes may be governed by statistics at the macroscopic scale, and controlled by mechanical stretching. (iv) Patterning. As noted previously, metalloporphenes might be used to build nanoscale circuits by inserting different metals in a specific pattern. Our results suggest that 0 UU or 0 U metalloporphenes could be used as a non-conductive blank canvas, which one could pattern by inserting metals forming 0 tt and 0 t metalloporphenes. (v) Magnetism. The potential for use of metalloporphenes in spintronics is currently speculative, as their magnetic ground state and exchange coupling is still unknown. Unless the exchange interaction through the π-system is very strong, the relatively large separation (~8.5 Å) between metallic centers will prevent fast decoherence, 48 which is essential for building multi-qubit devices. Fine control over the strength of the exchange coupling could be achieved by metal choice: for example, the metallic conductor 2 Cu(II)P will likely have a larger coupling than the semiconducting 2 Cr(III)ClP. Finally, the half-metallic behavior (i.e. conduction through only a single spin channel) of groups -1 (e.g. 2 V(III) + Cl2P -) and 1 suggests applications in spin filtering. Limitations. To paraphrase Imre Lakatos, the quality of a theory is determined by the accuracy of its predictions. 51 While hybrid DFT has undoubtedly been very successful in predicting properties of many materials, the procedure for choosing the ideal amount of exact exchange (or the Hubbard U) is not always clear, especially when geometry optimizations and large unit cells are involved, preventing the use of higher-level methods such as MP2 and GW. In case of metalloporphenes, using less than 35% EE yields a qualitatively wrong electronic structure (Figure ), while using a significantly larger amount overestimates localization, predicting too high spin multiplicities. For instance, PBE50 wrongly predicts the ground state of iron(II) porphyrin to be a quintet, while PBE0 and PBE38 correctly predict a triplet. 52 Therefore, our choice of the "best" amount of EE is bound on both sides, and inaccuracies resulting from our choice are likely limited to subgroups of 0 (e.g. ZnP is predicted to be in 0 t if 20-30% EE is used, and in group 0 tt with <20% EE, cf. Figure ). Neglect of spin-orbit coupling may be a larger issue, but its inclusion is very costly due to the loss of time-reversal symmetry, making it impractical to perform such calculations on a large scale.