Exploring the high sensitivity of DFT thermochemistry for protonation states of a ferredoxin model complex [CH3S4Fe2IIIS2H]−

Density functional theory (DFT) thermochemistry of 3d transition-metal complexes is well-known to be sensitive to the amount of exact Hartree-Fock exchange incorporated into the exchange-correlation functional. For example, relative energies of different protonation states of iron-sulfur complexes may vary by hundreds of kJ/mol among different DFT methods. In the present study, we examine the relative energies of four protonation isomers of the [CH3S4Fe2IIIS2H]- [2Fe-2S] ferredoxin model. Compared to many-body ab initio phaseless auxiliary-field quantum Monte Carlo with multi-Slater determinant trial wavefunctions and fully connected singles and doubles coupled-cluster with perturbative triples methods, the r2SCAN12-D4, B3LYP-D4, and B97-1-D3(OP) approaches perform the best. We also demonstrate that density-corrected DFT on top of KS-CCSD electronic densities provides reliable results with the r2SCAN functional. Moreover, the direct random phase approximation on top of the TPSSh, O3LYP, and r2SCAN12 hybrid functionals performs well.