To further investigate the performance and applicability of CASPT2-IPEA, two minimum energy and five MECI structures of cytosine (C 4 H 5 N 3 O) were obtained and compared with MRCI results. In Table VI, energies relative to the S 0 state are summarized. For the two minimum energy structures, relative energies of the S 0 and S 1 are shown. For MECIs, the nomenclature used in Ref. 78 is employed, and only relative energies of the crossing states are shown; for instance, the crossing states of "CI12" are S 1 and S 2 . In Ref. 78, three levels of MRCI calculations were performed. "MRCI1" included single-excitation CSF generated from the CAS, with the core 1s, σ orbitals, and one oxygen lone pair always remaining frozen. The next level "MRCIσπ1" further included the excitations from 14 σ orbitals and the second oxygen lone pair. The highest level "MRCIσπ2" included single excitations from A similar reversal between CASSCF and CASPT2 has been found in previous studies, and the order of the states can be sensitive to the choice of the theoretical level. For the same reason, geometry optimizations for the S 1 (π → π * ) state were performed with S 1 at CASPT2 (consistent with MRCI) and with S 2 at CASSCF.