Embedding‐theory‐based simulations using experimental electron densities for the environment

The basic idea of frozen‐density embedding theory (FDET) is the constrained minimization of the Hohenberg–Kohn density functional EHK[ρ] performed using the auxiliary functional , where ΨA is the embedded NA‐electron wavefunction and ρB(r) is a non‐negative function in real space integrating to a given number of electrons NB. This choice of independent variables in the total energy functional makes it possible to treat the corresponding two components of the total density using different methods in multi‐level simulations. The application of FDET using ρB(r) reconstructed from X‐ray diffraction data for a molecular crystal is demonstrated for the first time. For eight hydrogen‐bonded clusters involving a chromophore (represented as ΨA) and the glycylglycine molecule [represented as ρB(r)], FDET is used to derive excitation energies. It is shown that experimental densities are suitable for use as ρB(r) in FDET‐based simulations. For the first time, the use of experimentally derived molecular electron densities as ρB(r) in calculations based on frozen‐density embedding theory (FDET) of environment‐induced shifts of electronic excitations for chromophores in clusters is demonstrated. ρB(r) was derived from X‐ray restrained molecular wavefunctions of glycylglycine to obtain environment densities for simulating electronic excitations in clusters.