Calculations using different quantum mechanical methods including semiempirical (MNDO,AM1 and PM3), ab initio (RHF and MP2 calculations using the 6-311G and 6-311++G** basis sets), and density functional theory (LSDA, BP, MIXBP and B3LYP, i.e., B3LYP/6-311+G**//B3LYP/6-31G*) have been performed on the thermal fragmentation of cyclopropanone to ethylene and carbon monoxide. All RHF calculations predict a concerted single step mechanism for this conversion. The estimated activation energies vary from 34.4 to 54.6 kcal.mol(-1), mainly localized around 37+/-2 kcal.mol(-1), depending on the method. Whereas the calculated RHF reaction energies also varied from 14.5 to -33.3 kcal.mol(-1), the B3LYP/6-311+G**//B3LYP/6-31G* method predicts the experimental value (-17.7 kcal.mol(-1)) within experimental uncertainties. Remarkably, semiempirical AM1 and PM3 methods and simple DFT calculations, LSDA, predict comparable results to the more advanced methods. UHF ab initio calculations predict the same single step mechanism, whereas a multistep biradical mechanism with an unrealistically low activation energy is favored by the semiempirical methods. Structures of the activated complex of the single step mechanism, estimated by different methods, are very similar and consistent with a nonlinear cheletropic [(2)pi(s) + (2)omega(a)] reaction, as predicted by the orbital symmetry rules and earlier EHT calculations.