As long as the coupling between the CASPT2 density and the solvent is ignored, extending the method to CASPT2 is straightforward. The reaction field is determined at the SCF level, and during the PT2 calculation, the solvent effect is fixed and included in a perturbative manner. This approach is similar to that described in Ref. 59 and corresponds to the PTE approach. Although the solvent reaction field remains fixed during the PT2 calculation in this study as well, it is possible to relax the solvent after obtaining the fully relaxed (or unrelaxed) density matrix. The impact of this relaxation on the energetics is discussed in Section 4.3.2.