It is advantageous to examine the impact of using the CASPT2 density, comparing the PTE and PTED approaches. As described earlier, geometry optimizations at the CASPT2 level are performed with a fixed reaction field determined at the SCF level (PTE approach). However, single-point energies can be computed using a more sophisticated method: the reaction field can be determined at the CASPT2 level. The first-order correction to the wavefunction obtained in this way alters the electron density of the solute molecule, and the solvent charges can polarize in response to this perturbed density. Since the wavefunction and solvent charges are non-linearly coupled, the free energy fully coupled with the CASPT2 density is obtained iteratively. This method is referred to as the PTED approach. Although the variational (fully relaxed) density can be obtained during the analytic gradient calculation, the unrelaxed density [excluding the response contribution of the wavefunction at the SCF level (κ γ,[ϕ ] p,q and P γ,[ϕ ] η,π )] is used to compute the PTED energy. Table summarizes the evolution of the equilibrium free energy at both the ground (S 0 ) and charge-transfer excited states (S 1 ) in water. The energy at the zeroth iteration corresponds to the PTE energy, while the energy at convergence represents the PTED energy. ϕ (ϕ = 0, 1) (unit in hartree) and the decrease of the energy ∆G ϕ (unit in eV) in water as a function of the iteration count at (SA5-)XMS-CASPT2(16e,12o)/ANO-RCC-VTZP.