The corresponding line can be considered an approximate phase-change line, and such a result for polyenes is shown in Figure . Additionally, we can compare the quantum-chemically computed Q max values with the quantum-chemically parameterized models, both based on simple Hückel's method and more general parametrizations with hardness given by Equation . The results are shown in Figure . The trend for the polyenes from Hückel's method is too fast-growing, which is attributed to the HOMO-LUMO gap approaching zero. The empirically-fitted hardness gives better trends, although lower in energy than the quantum-chemical values. This is probably due to approximating the parent species and using their values for the fragments. In the case of linear polyenes, this approximation gives better results since the products are reminiscent of the parent species. However, in the case of the annulene species, the resulting fragments are structurally different from those from polyenes, and these effects, like differences in trans-/cis-energies, can lead to the observed deviations. Nevertheless, we can say that the approximate model based on the hardness of the reagents and products reproduces the important features of the size-maximal charge phase diagrams.