Spin-State Energetics of Heme-Related Models from DFT and Coupled Cluster Calculations.

Spin-state energetics of metalloporphyrins and heme groups is elucidated by performing high-level coupled cluster calculations for their simplified mimics. An efficient computational protocol is proposed-based on the mix of extrapolation to complete basis set and explicitly correlated (F12) methodology-which retains the high accuracy of the CCSD(T) method at a cost that makes it applicable also to relatively large models, e.g., FeP and FeP(Cl) (P = porphin). Adequacy of CCSD(T) is supported by analysis of multireference character and comparison with the completely renormalized CR-CC(2,3) method. The high-level coupled cluster results are used for assessment of density functional theory (DFT) methods, for which an accurate description of the spin-state energetics is recognized as a major challenge. Although the DFT results are highly functional-dependent, it is shown that the spin-state energetics of a full heme model and its simplified mimic remain in a good linear correlation. This makes it possible to estimate the spin-state energetics of full heme models based on the accurate CCSD(T) results for their mimics, as illustrated for porphyrin complexes of Fe(II), Mn(II), and Co(II); pentacoordinate heme complexes of Fe(II) and Fe(III); and a ferryl heme model. Comparison with the available experimental data is also presented.