  The Frenkel exciton model was adapted to describe X-ray absorption and
resonant inelastic scattering spectra of polynuclear transition metal complexes
by means of restricted active space self-consistent field method. The proposed
approach allows to substantially decrease the requirements to computational
resources if compared to a full supermolecular quantum chemical treatment. This
holds true in particular in cases where the dipole approximation to the
electronic transition charge density can be applied. The computational protocol
was applied to the calculation of X-ray spectra of the hemin complex, which
forms dimers in aqueous solution. The aggregation effects were found to be
comparable to the spectral alterations due to the replacement of the axial
ligand by solvent molecules.
