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