Can Semilocal Approximations to the Embedding Potential Tackle Charge-Transfer-to-Solvent Excitations? An Aqueous Thiocyanate Example

Frozen-density embedding theory (FDET) provides a formal basis for methods employing density-dependent embedding potentials. FDET-based methods involve approximations concerning: (i) the choice of the approximant for the bifunctional vxctnad[ρA,ρB], (ii) the localization of the embedded wave function, and (iii) the approach used to generate the electron density of the environment. The set of approximations that has been shown to yield highly accurate complexation-induced shifts in vertical excitation energies for valence excitations localized on the chromophore─referred to as the "standard FDET protocol"─is expected to fail if applied to charge-transfer-to-solvent excitations. This work illustrates that such excitations can also be treated using an FDET-based method; however, they require refinement of the approximations used in the standard protocol.