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

Abstract

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.