Following the strategy outlined above, we built OX-B3LYP (Optimized for eXtensively conjugated systems, dark blue in Figure ) as the optimal DFA for describing the c-PN nanobelts. At small r12, OX-B3LYP (EE0 = 19%) is almost identical to B3LYP (EE = 20%), which is a suitable DFA for modelling the optical properties of edge-fused porphyrin nanoribbons l-PN (Figure ; see SI Section B). In the long-range limit, OX-B3LYP recovers the correct form of the Columbic decay (EE∞ = 100%).