We tested three semiempirical methods, DFTB3, PM6 and GFN2-xTB, and three versions of the Perdew-Becke-Ernzerhof functional, PBE, PBE0, and LC-PBE, to optimize the transition state structure leading to the ozonide in the first part of the method benchmarking step. We did not employ CC methods to optimize molecular structures because calculations would become too slow. Figure shows the the results of structure and energy benchmarks that we conducted to identify the optimal quantum chemistry methodology for ensuing CRN explorations. The data in Figure demonstrate that no single quantum chemistry method simultaneously offers low deviations in molecular structures and electronic energies. Therefore, Figure : Ozonolysis reaction of ethene and tetramethylethene through the ozonide intermediate (1). This Criegee mechanism 70 proceeds either directly with an heterolytic cleavage forming a carbonyl compound and an alkyliumperoxolate (3) or via a zwitterionic intermediate (2), the latter leading to aldehyde and α-hydroxyl-alkylperoxide (5) in presence of water as solvent, and to the Criegee ozonide (4) in apolar solvents. it is necessary to define a composite approach, where molecular structures are optimized first with one method, and electronic energies are subsequently calculated with a second method.