State-Specific Embedding Potentials for Excitation-Energy Calculations

Embedding potentials are frequently used to describe the effect of an environment on the electronic structure of molecules in larger systems, including their excited states. If such excitations are accompanied by significant rearrangements in the electron density of the embedded molecule, large differential polarization effects may take place, which in turn can require state-specific embedding potentials for an accurate theoretical description. We outline here how to extend wave function in density functional theory (WF/DFT) methods to compute the excitation energies of a molecule in a responsive environment through the use of state-specific density-based embedding potentials constructed within a modified subsystem DFT approach. We evaluate the general expression of the ground- and excited-state energy difference of the total system both with the use of state-independent and state-dependent embedding potentials and propose some practical recipes to construct the approximate excited-state DFT density of the active part used to polarize the environment. We illustrate these concepts with the state-independent and state-dependent WF/DFT computation of the excitation energies of <i>p</i>-nitroaniline, acrolein, methylenecyclopropene, and <i>p</i>-nitrophenolate in various solvents