  Iron-sulfur complexes play an important role in biological processes such as
metabolic electron transport. A detailed understanding of the mechanism of long
range electron transfer requires knowledge of the electronic structure of the
complexes, which has traditionally been challenging to obtain, either by theory
or by experiment, but the situation has begun to change with advances in
quantum chemical methods and intense free electron laser light sources. We
compute the signals from stimulated X-ray Raman spectroscopy (SXRS) and
absorption spectroscopy of homovalent and mixed-valence [2Fe-2S] complexes,
using the {\it ab initio} density matrix renormalization group (DMRG)
algorithm. The simulated spectra show clear signatures of the theoretically
predicted dense low-lying excited states within the d-d manifold. Furthermore,
the difference in signal intensity between the absorption-active and
Raman-active states provides a potential mechanism to selectively excite states
by a proper tuning of the excitation pump, to access the electronic dynamics
within this manifold.
