How Accurately Can Extended X‑ray Absorption Spectra Be Predicted from First Principles? Implications for Modeling the Oxygen-Evolving Complex in Photosystem II

First
principle calculations of extended X-ray absorption fine
structure (EXAFS) data have seen widespread use in bioinorganic chemistry,
perhaps most notably for modeling the Mn4Ca site in the
oxygen evolving complex (OEC) of photosystem II (PSII). The logic
implied by the calculations rests on the assumption that it is possible
to a priori predict an accurate EXAFS spectrum provided
that the underlying geometric structure is correct. The present study
investigates the extent to which this is possible using state of the
art EXAFS theory. The FEFF program is used to evaluate the ability
of a multiple scattering-based approach to directly calculate the
EXAFS spectrum of crystallographically defined model complexes. The
results of these parameter free predictions are compared with the
more traditional approach of fitting FEFF calculated spectra to experimental
data. A series of seven crystallographically characterized Mn monomers
and dimers is used as a test set. The largest deviations between the
FEFF calculated EXAFS spectra and the experimental EXAFS spectra arise
from the amplitudes. The amplitude errors result from a combination
of errors in calculated S02 and Debye–Waller values as well as uncertainties in background
subtraction. Additional errors may be attributed to structural parameters,
particularly in cases where reliable high-resolution crystal structures
are not available. Based on these investigations, the strengths and
weaknesses of using first-principle EXAFS calculations as a predictive
tool are discussed. We demonstrate that a range of DFT optimized structures
of the OEC may all be considered consistent with experimental EXAFS
data and that caution must be exercised when using EXAFS data to obtain
topological arrangements of complex clusters.