Figure shows the most favorable reaction mechanism from ozone and ethene (source) to α-hydroxymethylhydroperoxide (target). The other 24 alternative mechanisms are depicted in grey because Pathfinder classifies them as unlikely due to their greater compound costs (see elementary steps of reaction paths in Section S2.6.1). Mechanism 1 depicted in Figure is spontaneous, with a total reaction Gibbs energy of -365 kJ•mol -1 . This reaction pathway starts with the barrierless formation of the vdW complex (Int1 in Figure ). Despite the fact that the Pathfinder mechanism is not the lowest in energy (there are other pathways in Figure with lower energy), it is indeed the shortest and the most favorable when considering both the activation barriers (i.e., kinetics) and initial concentrations of the reactants. Moreover, the alternative mechanisms show that there is a consistent trend in terms of total reaction Gibbs energy. For example, mechanism 25 (see Section S2.6.1) has a cost of 159.9 compared to 124.8 for mechanism 1. The difference stems from the fact that mechanism 25 includes an additional reaction step. For CRN-E, the formation of the α-hydroxymethylhydroperoxide is exergonic for all the reaction pathways shown in Figure .