The comparison between RHF and UMP2 optimizations, for the ene reaction, indicates that RHF level calculations can reproduce the transition state geometries of the higher level of theory accurately. Also the UMP2//RHF activation energies are in very good agreement with UMP2//UMP2 results. The relatively inexpensive UMP2/6-31G∗//RHF/3-21G calculation for the reaction between propene and maleic anhydride reproduces the experimental activation energy within the experimental uncertainty. The semiempirical AM1 method is suitable for preliminary studies of the ene reaction, but the UHF/AM1 level results should be taken with caution.
Yliniemelä, A., Konschin, H., Pietilä, L-O., & Teleman, O. (1994). A comparative study on energetics and mechanism of the Ene reaction. Journal of Molecular Structure, 334(2-3), 173-177. https://doi.org/10.1016/0166-1280(94)04030-V