However, this approach is applicable only when f γ pq is constructed with the state-averaged density matrix (i.e., SS-CASPT2 with single-state CASSCF reference or XMS-CASPT2 using all internal states). In this study, our aim was to develop analytic derivatives using state-dependent zeroth-order Hamiltonian (the original MS-CASPT2, XDW-CASPT2, and RMS-CASPT2), so we instead determine κ act,γ,αβ tu by the iterative method introduced by Song et al. for CASPT2. First, we take the partial derivative of L CASPT2 γ,αβ with respect to κ γ pq , P γ⊥ δ∆ , and κ act,γ tu and obtain the matrix linear equation: