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 E_{v_{AB}}^{\rm FDET}[\Psi _A, \rho _B], 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 E_{v_{AB}}^{\rm FDET}[\Psi _A, \rho _B] 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.