and Best models. In order to investigate the effect of entrance and exit van der Waals wells, two multi-step reactions of astrochemical (CN addition to ethylene) and atmospheric (reaction between the simplest Criegee intermediate and H 2 O) interest have been investigated. Aminoacetonitrile (AN), also known as vinylcyanide, has been found in several regions of the interstellar medium (ISM) and may be also the best candidate for the formation of celllike membranes in Titan's hydrocarbon-rich lakes and seas. Among the possible formation routes of AN, we have considered the addition of CN radical to ethylene, since both species are present in the ISM and on Titan. From an experimental point of view, the reaction was found to be very fast, approaching the gas kinetics limit at very low and very high temperatures. On the other hand, the computational studies performed till now did not employ state-of-the-art quantum chemical models. The reaction mechanism is sketched in Figure together with the junChS-F12 relative energies (including rDSD anharmonic ZPEs) of all the stationary points. The rate constant for the addition issued from junChS-F12 computations is compared in Figure with the CBS-QB3 and Best counterparts. The addition rate constant is essentially flat in the whole temperature range and the different composite methods provide comparable results. Although a fully quantitative comparison with experimental rate constants is not possible because we have considered only the high pressure limit, our values have the correct order of magnitude, especially at high temperatures. Criegee intermediates (CIs) are carbonyl oxides formed in the ozonolysis of unsaturated hydrocarbons and play a central role in several processes occurring in the atmosphere. Reaction with water is a key step for several processes involving CIs and has been investigated in a number of studies. In particular, the rate constant for the reaction of the simplest CI (CH 2 OO) has been analyzed in a thorough computational study. The reaction mechanism is sketched in Figure