Density functional theory and coupled cluster calculations were performed with the ORCA software package version 5.0.3. We also tested three semiempirical methods, namely DFTB3, 43 PM6 44 and GNF2-xTB. We have not considered the application of multireference configuration interaction methods because of the two following limitations. First, molecular orbitals that form the complete active space must be consistent for all the molecular structures along a reaction path, hence new methodologies are currently under development to provide direct orbital selection mapping. Second, multireference methods are notorious for being among the most time-demanding calculations in quantum chemistry. Given that pioneering studies demonstrated that CC succeeded in both optimizing the molecular structures and predicting the reaction energies of the Criegee mechanism for ozone and ethene, we have followed their recommendation. DFT structure optimizations were done with Perdew-Becke-Ernzerhof PBE, the hybrid PBE0, and the long-range LC-PBE 50 density functionals, with the Ahlrichs def2-TZVP basis set. Grimme's D3 dispersion correction was employed for the PBE and PBE0 calculations. Single point calculations were performed with canonical coupled cluster, CCSD(T), and the Domain-based Local Pair Natural Orbital approximation of coupled cluster, DLPNO-CCSD(T) with the correlation consistent basis sets. Because the reaction network contains both open-and closed-shell compounds, we employed the DLPNO approach, instead of the DPNO, to ensure that we can combine their absolute electronic energies when calculating reaction energies. We employed the default protocol in ORCA, where UHF alpha and beta orbitals are transformed to quasi-restricted orbitals, thus removing much of the spin contamination. Solvation in water was introduced using the Conductor-like Polarizable Continuum Model, CPCM, both for structure optimization and single point calculations. Stationary points were characterized with analytic frequency calculations. Gibbs energy corrections were computed using LC-PBE, at 298.15 K and 1 atm, and using the ideal gas-rigid rotor-harmonic oscillator (IGRRHO) model. Final Gibbs energies were calculated combining the electronic energy from DLPNO-CCSD(T), and the Gibbs energy corrections from the DFT method. In order to employ molar concentrations in the rate constant, the reference state was modified to 298.15 K and 1.0 M (standard state in solution). Given that the temperature remains constant between both states, this was done by simply increasing the barrier by 7.90 kJ•mol -1 .