CASPT2-IPEA predicted lower absorption energies than MRCI (5.136 eV at MRCIσπ2 Regarding the relative energies of MECIs, CASSCF failed to reproduce the energetic order between CI01 sofa and CI01 twist , which can be important MECIs for ultrafast radiationless decay of cytosine. These MECIs cross between the closed-shell singlet and π → π * states. By contrast, for higher-state MECIs, in particular, CI12 ′ (crossing between π → π * and n → π * ) and CI23 (n O → π * and n N → π * ), CASSCF could predict the relative energies rather well, and the deviation from the reference MRCI result was approximately 0.1 eV. On the other hand, with CASPT2, the deviation from the reference MRCI result for CI01 twist is larger than the expected accuracy of MRPT. However, the average energies of S 0 and S 1 in the single-point energy calculation at a higher level, MRCIσπ2, 78 were 4.269 and 3.980 eV for CI01 sofa and CI01 twist , respectively, so CASPT2-IPEA was close to the expected accuracy. The energy gaps between S 0 and S 1 in the single-point calculation at MRCIσπ2 were as large as 0.455 and 0.166 eV, 78 respectively; accurate estimation of the relative energy for these MECIs seems to be challenging, and it would be desirable for geometries and energies to be obtained with even higher-level calculations. For higherstate MECIs (CI12 [crossing between π → π * and n O → π * ], CI12 ′ , and CI23), CASPT2 (in particular RMS-CASPT2) without IPEA shift predicted much lower energies than the reference MRCI, and the deviations could be much larger than those of CASSCF. On the other hand, CASPT2-IPEA (in particular, RMS-CASPT2-IPEA) predicted quite well with a deviation less than 0.05 eV for the higher-state MECIs.