  The basic idea of Frozen-Density Embedding Theory (FDET) is the constrained
minimisation of the Hohenberg-Kohn density functional $E^{HK}[\rho]$ performed
using the auxiliary functional $E_{v_{AB}}^{FDET}[\Psi_A,\rho_B]$, where
$\Psi_A$ is the embedded $N_A$-electron wave-function and
$\rho_B(\vec{\mathrm{r}})$ 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 $E_{v_{AB}}^{FDET}[\Psi_A,\rho_B]$ makes it possible to
treat the corresponding two components of the total density using different
methods in multi-level simulations. We demonstrate, for the first time, the
applications of FDET using $\rho_B(\vec{\mathrm{r}})$ reconstructed from X-ray
diffraction data on a molecular crystal. For eight hydrogen-bonded clusters
involving a chromophore (represented with $\Psi_A$) and the glycylglycine
molecule (represented as $\rho_B(\vec{\mathrm{r}})$), FDET is used to derive
excitation energies. It is shown that experimental densities are suitable to be
used as $\rho_B(\vec{\mathrm{r}})$ in FDET based simulations.
