| Summary: | The calculation of the heats of combustion ΔH°<sub>c</sub> and formation ΔH°<sub>f</sub> of organic molecules at standard conditions is presented using a commonly applicable computer algorithm based on the group-additivity method. This work is a continuation and extension of an earlier publication. The method rests on the complete breakdown of the molecules into their constituting atoms, these being further characterized by their immediate neighbor atoms. The group contributions are calculated by means of a fast Gauss–Seidel fitting calculus using the experimental data of 5030 molecules from literature. The applicability of this method has been tested by a subsequent ten-fold cross-validation procedure, which confirmed the extraordinary accuracy of the prediction of ΔH°<sub>c</sub> with a correlation coefficient R<sup>2</sup> and a cross-validated correlation coefficient Q<sup>2</sup> of 1, a standard deviation σ of 18.12 kJ/mol, a cross-validated standard deviation S of 19.16 kJ/mol, and a mean absolute deviation of 0.4%. The heat of formation ΔH°<sub>f</sub> has been calculated from ΔH°<sub>c</sub> using the standard enthalpies of combustion for the elements, yielding a correlation coefficient R<sup>2</sup> for ΔH°<sub>f</sub> of 0.9979 and a corresponding standard deviation σ of 18.14 kJ/mol.
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