Choice of electronic structure method
From the above discussion, one may wonder whether complete active space
methods are better suited to represent the relevant electronic
configurations, as they are in principle able to represent
multireference cases and low-lying excited states. One study in this
direction is briefly mentioned, although as stated in the introduction,
active space approaches will have prohibitive computational cost for
their applications in screening problems such as that for the active
site of FeNC catalysts.
Sadoc et al.77 used CASSCF/CASPT2 wavefunctions to
predict Mössbauer isomer shifts. They found a good correlation between
the isomer shift and ρ (0) as the sum of natural orbital densities
at the point of the nucleus multiplied with the natural orbital
occupation numbers (R2=0.984). No direct correlation
between the effective d-electron count and the isomer shift was found,
in line with previous assessments based on density functional
theory.14 Instead, the isomer shift correlated with
the covalency of the Fe-ligand bonds –taken as the difference between
the formal and the computed charge of the iron ion– and was interpreted
as a measure of the deviation from the ionic model.