## 2 Experimental Data and Computational Methods
### Computational Methods
[19], who have reported the results of all their calculations in the supplementary material to their paper and in a database which is freely accessible on the Virtual Chemistry website; for further details, see Supplementary Materials S2. Based on the existing literature, the G4 quantum method seems to provide comparatively accurate values for the heat of formation of organic molecules as presented in various studies [19; 20; 21; 22; 23], and therefore we will also report G4 results in the present paper when we have reliable experimental data. The latter provide further validation of the G4 method and allow us to provide heats of formation for species for which we have no reliable experimental data available. Contrary to other works in which the G4 method was used to evaluate the heat of formation, in the current work, whenever possible, we computed the averaged value for the heat of formation over different accessible conformations. It is formally more correct and has an impact compared to minimum energy structure evaluation only when flexible units are present. This can be nicely demonstrated using the G4 results reported by [PERSON] c.s. [19] for the alkanes, as shown in Table S3 in the Supplementary Materials. For hexadecane, the difference between G4 and experimental results has increased to about 7 kJ/mol and the increment per CH\({}_{2}\) unit is \(-21.4\) kJ/mol, whereas in reality the increment based on experimental values is close to \(-20.6\) kJ/mol. Exactly the same is observed for the n-alcohol data from [PERSON]'s paper. In our previous paper [4], we observed, from calculations on substituted cycloalkanes, that the contribution due to conformational averaging is typically of the order of 1 kJ/mol depending on the structure and therewith the accessible conformations. In addition to G4 calculations, we incidentally invoked B3 LYP calculations with a 6-311+C** basis set to evaluate _relative_ energies within a series of molecules. This is an older density functional, being a mix of density functional character and the traditional Hartree-Fock character, which turned out to give good relative energies for organic molecules [29]. We also used w-B97X-D with a 6-311++C** basis set, as this functional was claimed to be significantly superior for non-bonded interactions [30]. These density functional type (DFT) quantum calculations were performed using the Spartan program [31]. This type of calculations were only and specifically applied to evaluate relative energies between different configurations of the same molecule, and subsequently employed to determine whether certain differences in heat of formation between specific molecules can be rationalized before introducing additional Group Contribution parameters. Even though the B3 LYP method has always been considered an appropriate method to evaluate energy differences between organic species, we emphasize that, as some readers may argue that another method may lead to more accurate results, we only use the results in an indicative way whereas the numerical values of the Group Contribution parameters were always based on either, preferably, experimental results or on G4 calculated results.