As a first example, two constitutional isomers with the formula C 5 H 8 O, namely an α, β-(3-penten-2-one) and a β, γ-unsaturated enone (4-penten-2-one) is considered (see Fig. ). It is noticeable that 3-penten-2-one has a smaller gap compared to 4-penten-2-one (by 1.12 eV) because delocalization results in a greater mobility of π-electrons throughout the molecular structure. However, both molecules present a similar polarizability coming from the identical atomic composition as well as similar total size.