The most favorable reaction mechanism from ozone and tetramethylethene to 2-hydroxy-2-propylhydroperoxide is shown in Figure . The 24 alternative mechanisms are depicted in grey and exhibit greater compound costs. For instance, mechanism 25 illustrated in Section S2.6.2 has a cost of 288.5 as compared to 104.4 for mechanism 1. This large difference is not only caused by the extra reaction step of mechanism 25, but also by the formation of hydrogen which is not observed in experiments. Mechanism 1 starts with the barrierless formation of the vdW complex (Int1) and then undergoes a 1,3-dipolar cycloaddition through TS1 with an activation energy of 4 kJ•mol -1 to generate the ozonide (Int2). Note that the activation barrier for TS1 from tetramethylethene is 30 kJ•mol -1 lower than TS1 from ethene, in good agreement with the higher reactivity of tetramethylethene reported in experiments (see k exp CRN-E, it is not for CRN-T. This interpretation perfectly agrees with the experimental data, as α-hydroxymethylhydroperoxide is one of the main products whereas 2-hydroxy-2propylhydroperoxide is not.