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.