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 E HK[ρ] performed using the auxiliary functional , where Ψ A is the embedded N A -electron wavefunction and ρ B (r) is a non-negative function in real space integrating to a given number of electrons N B. 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 ρ B (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 Ψ A ) and the glycylglycine molecule [represented as ρ B (r)], FDET is used to derive excitation energies. It is shown that experimental densities are suitable for use as ρ B (r) in FDET-based simulations.