and the MO coefficient matrix is obtained by block-diagonalizing f γ pq (quasi-canonical MOs). Thus, for SS-CASPT2, MS-CASPT2, RMS-CASPT2, and XDW-CASPT2, N state sets of the MO coefficient matrix are defined. XMS-CASPT2 and RMS-CASPT2 are obtained by taking the limits ζ → 0 and → ∞, respectively; therefore, in the following derivation, we use the zeroth-order Hamiltonian of XDW-CASPT2, although no XDW-CASPT2 calculations are performed in this study.