As mentioned in the previous section, the computed results depend on the number of states (N state ) included in the model space, and this effect is herein discussed for XMS-, XDW-, and RMS-CASPT2(6e,4o) using ethylene. For this system and active space, the number of CSFs is ten, and calculations up to the complete model space can be performed. In the excitation energy depends on the number of states for the three methodologies can be rationalized as follows. Because the reference energy (i.e., the one used for ∆ αβ in Eq. ( )) of S 0 is very different from that of the excited states, the zeroth-order Hamiltonian in XDW has a very pronounced state-specific character, which makes it similar to that of RMS. In fact, the absolute S 0 energy for both of these methods differ by at most 0.045 eV, and their ground state properties are therefore expected to be comparable. In contrast, S 0 from XMS is considerably different, because the Fock operator is averaged over all states with equal weights regardless of the gap between the ground and the excited states. The situation is different for S Hence, even though XDW is formally interpolating between XMS and RMS in terms of the ζ parameter, the predicted relative energies will generally not interpolate between these two extremes.