Potential energy surfaces, minimum energy reaction paths, minima, transition states, reaction barriers, and conical intersections for the most important atmospheric reactions of methyl nitrate (CH3ONO2) and methylperoxy nitrite (CH3OONO) on the electronic ground state have been studied (i) with the second-order multiconfigurational perturbation theory (CASPT2) by computation of numerical energy gradients for stationary points and (ii) with the density functional theory (DFT). The proposed mechanism explains the conversion of unreactive alkyl peroxy radicals into alkoxy radicals: CH3O2 + NO reversible arrow CH3OONO reversible arrow CH3O reversible arrow NO2 reversible arrow CH3ONO2. Additionally, several discrepancies found in the comparison of the results obtained from the two employed approaches are analyzed. CASPT2 predicts that all dissociation reactions into radicals occur without an extra exit energy barrier. In contrast, DFT finds transition states for the dissociations of cis- and transmethylperoxy nitrite into CH3O + NO2. Furthermore, niulticonfigurational methods [CASPT2 and complete active space SCF (CAS-SCF)] predict the isomerization of CH3ONO2 to CH3OONO to occur in a two-step mechanism: (i) CH3ONO2 -> CH3O + NO2; and (ii) CH3O + NO2 -> CH3OONO. The reason for this has to do with the Coupling of the ground electronic state with the first excited state. Therefore, it is demonstrated that DFT methods based on single determinantal wave functions give an incorrect Picture of the aforementioned reaction mechanisms.