Peroxylradicals(RO2) are importantintermediatesin the atmosphericoxidationprocesses.TheRO2can reactwithotherRO2to formreactiveintermediatesknownas tetroxides,RO4R. Thereactionmechanismsof RO4R formationanditsvariousdecompositionchannelshavebeeninvestigatedin multiplecomputationalstudies,but previousapproacheshavenot beenableto providea unifiedmethodologythatis ableto connectallrelevantreactionstogether.An apparentdifficultyin modelingtheRO4R formationand its decompositionis the involvementof open-shellsingletelectronicstatesalongthe reactionpathway.Modelingsuchelectronicstatesrequiresmultireference(MR)methods,whichwe use in the presentstudyto investigatein detaila modelreactionof MeO2+ MeO2→MeO4Me, and its decomposition,MeO4Me→MeO+ O2+ MeO,as well as the two-bodyproductcomplexesMeO···O2+ MeOand MeO···MeO+ O2. The usedMRmethodsare benchmarkedagainstrelativeenergiesof MeO2+ MeO2, MeO4Me, and MeO+ MeO+ O2, calculatedwith CCSD(T)/CBS,W2X,and W3X-Lmethods.We foundthat the calculatedrelativeenergiesof the overallMeO2+ MeO2→MeO4Me→MeO+ O2+ MeOreactionare verysensitiveto the chosenMR methodand that the CASPT2(22e,14o)-IPEAmethodis abletoreproducethe relativeenergiesobtainedby the variouscoupled-clustermethods.Furthermore,CASPT2(22e,14o)-IPEAand W3X-Lresultsshowthat the MeO···O2productcomplex+ MeOis lowerin energythanthe MeO···MeOcomplex+ O2. The formationofMeO4Me is exothermic,and its decompositionis endothermic,relativeto the tetroxide.Boththe formationand decompositionreactionsappearto followpathwayswithno saddlepoints.Accordingto potentialenergysurfacescansof the decompositionpathway,the concertedcleavageof bothMeO···Obondsin MeO4Me is energeticallypreferredoverthe correspondingsequentialdecomposition.