From optical spectroscopy, it is known that the optical gap is primarily determined by the functional groups in a molecule. This is reflected in characteristic optical gaps (vertical excitation energies of the lowest electronic transitions) per functional groups, e.g., the ππ * absorption of an isolated alkene-group as chromophore is between 7.51 and 6.70 eV. Since we are assuming that HOMO-LUMO gap is a good starting point in determining the optical gap of a molecule, it would be expected to find that ∆E HL values are also clustered by certain functional groups. In contrast, our analysis has shown that polarizability (α) is primarily determined by the atomic composition of a molecule. To draw more general conclusions about these properties, we analyse the two-dimensional (2D) property space defined by HOMO-LUMO gap and polarizability for a selected subset of QM7-X molecules 8 (see Sec. 6.2). This dataset enables us to study the (∆E HL , α) relationship more broadly because it covers a considerable number and variety of chemical compounds. Fig. (a) shows the (∆E HL , α)-space for the QM7-X molecules -indicating no direct relationship between the two quantities across the chemical compound space spanned by this dataset (R 2 = 0.13). Furthermore, the role of the two main factors that determine ∆E HL (functionality) and α (atomic composition) are highlighted in Fig. show a value of circa 6.5 eV while, for primary alcohol group, they extend from 7.2 eV to 7.8 eV.