## 4 Discussion and Conclusions  
In the previous four papers in this series, we developed a GC parametrization for various classes of organic molecules whilst, for the overwhelming majority of molecules considered, achieving unprecedented chemical accuracy. In the present paper, we considered further chemical Groups with the aim of extending the range of applicability of our GC approach. While advancing considering more Groups, the availability of reliable experimental data becomes an ever increasing issue. Again, the critical selection of experimental data from verified collections was crucial, and when data from another source were taken, this generally led to larger discrepancies between the model and experimental values. The new classes of molecules include amines, alkylesters, various substituted benzenes, including t-butyl-benzenes, phenol, methoxybenzenes, anilines, benzaldehydes, and acetophenones, and finally furans and indoles/indolines. The Group parameters previously established are collected in Supplementary Materials S4, accompanying the previous publication [4]. An update is provided in Supplementary Materials S4 to the current paper and also includes a few explicit calculations of the heat of formation according to our GC model. In several cases, the chemical accuracy criterion could not be met. Firstly, this concerns methylamines for which we find that the GC model value is generally more negative than the experimental value. The experimental value is generally in good agreement with the G4 quantum calculated value. However, in the case of N-methyl-butanamine, but also for butylisobutylamine which does not contain a methyl substituent, we observe that \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Indoles** & **Exp.** & **Model** & **Model – Exp.** & **G4 – Exp.** & **G4** & **MG in ICAS23** \\ \hline indole & 162.2 & & & \(-1.8\) & 160.4 & 213.1 \\ \hline
2-methylindole & 121.6 & 119.8 & \(-1.8\) & \(-1.4\) & 120.2 & 118.8 \\ \hline indoline & 120.8 & & & \(-3.2\) & 117.6 & 120.8 \\ \hline
2-methylindoline & 80.2 & 78.4 & \(-1.8\) & \(-0.8\) & 79.4 & 80.4 \\ \hline
1-methylindole & 152.1 & 153.3 & 1.2 & \(-2.2\) & 149.9 & 161.0 \\ \hline
1-methylindoline & 100.8 & 111.9 & 11.1 & 5.3 & 106.1 & 101.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 17: Experimental, present GC model, and G4 model valuers for the heat of formation for indoles and indolines. All values in kJ/mol. Experimental and G4 data for the first four entries are from Verevkin c.s. [50] and for the 1-methyl substituted species from Ref. [51]. Indole and indoline are, within our GC model, considered as a Group and therefore there are no model values in the table as these are taken to be identical to the experimental values. Figure 2: Chemical structures of indole (**left**) and indoline (**right**). the GC model value is more positive rather than more negative than the experimental value. But, more interestingly, for these cases, we observe that the G4 value is in good or better agreement with the GC model value and the G4 values reveal a significant discrepancy between the experimental values. The good performance of the G4 method in general might imply a larger error in the experimental values. Here, we note that some experimental values we have referred to are in fact averages of several experimental results, recognizing that the gas phase heat of formation is not measured directly but often derived from thermodynamic quantities obtained on the liquid phase. Now, we have touched on the aspect of the quality and reliability of G4-evaluated heat of formation, following earlier works by [PERSON] c.s. (for references, see above) and our previous work in Refs. [4; 20; 21], we have continued to apply the G4 quantum method, and in the present work experienced that it tends to show good agreement, often within chemical accuracy, with the available experimental data.