corresponds to applying Equation with N/2 atoms and polyene parameters, given above. We note that this assumption of symmetric dissociation may fail if stable products are formed, like in the case of acetylene-like fragments. In particular, we observe this for adiabatic dissociation energies of the all-trans-1,3,5-hexatriene (section 1.3 of SI). However, the numerical tests show that the dissociation energy trends correlate well for the lowest energy asymmetric dissociation and the symmetric energy dissociation channels.