γ . The key difference between the XMS-, XDW-, and RMS-CASPT2 methods lies in how the generalized Fock operator (Eq. ( )) is defined. XMS-uses the equally state-averaged density matrix, XDW-uses a dynamically weighted average, and RMS-employs a state-specific density matrix. To compute the second-order perturbation energy, the first-order correction to the wavefunction Ψ