MS-CASPT2 suffers from the non-invariance problem and has been considered to be unsuitable for finding MECIs. As shown in past studies, the PES described with MS-CASPT2 can be rough, in particular, near CIs at CASSCF. Sources of the non-invariance have been solved using the modification by Shiozaki 51 based on the idea of XMCQDPT2 by Granovsky. As the IPEA shift introduces another source of non-invariance, it is useful to check the quality of PESs described with CASPT2-IPEA and to confirm that the PESs do not have (significant) irregularities. Close investigation indicates that the discontinuity can be attributed to the difference in the zeroth-order Hamiltonian rather than the use of IPEA shift. XMS-and RMS-CASPT2 use the state-averaged and state-specific density matrix, respectively, for defining the Fock matrix. The use of the state-specific density matrix can be an additional source of noninvariance, so it is natural that RMS-CASPT2 can give less smooth PESs. Nevertheless, the PES is overall smooth, and the discontinuity is noticeable only at one point, so both XMS-and RMS-CASPT2 can be used for locating MECIs. A similar argument applies to the right panel; there are no irregularities in the energy difference described with XMS-CASPT2, whereas there are two (∼4 kcal/mol) with RMS-CASPT2. In spite of these small irregularities, there were no (significant) problems in determining minimum energy and MECI structures in the later sections.