The main flaw of the Hückel's method solution is that the HOMO-LUMO gap of the extended systems decreases to zero upon increasing the size of the polyene/annulene. As we can see from quantum chemistry (Figure ), η decreases in the case of both the polyenes and annulenes, but not to zero, as predicted by Hückel's model, but to some finite value. To remove this limitation, we use a three-parametric expression for approximating quantum-chemical values to obtain the carbon backbone length-dependent graphs: where N is the number of carbon atoms in the molecule, and the three fitting parameters are η ∞ , η 0 , and K. K was limited to integer numbers. The results of the fitting are η With the hardness of the form given by Equation , the most energetically favorable breakdown channel is dissociation into two equal fragments (see Appendix for proof). In this case, when a polyene or annulene with N carbon atoms dissociates into two polyenes with N/2 carbons, we get the reduced hardness (Equation ) to be