Multireference protonation energetics of a dimeric model of nitrogenase iron-sulfur clusters

Characterizing the electronic structure of the iron--sulfur clusters in nitrogenase is necessary to understand their role in the nitrogen fixation process. One challenging task is to determine the protonation state of the intermediates in the nitrogen fixing cycle. Here, we use a dimeric iron--sulfur model to study relative energies of protonation at C, S or Fe. Using a composite method based on coupled cluster and density matrix renormalization group energetics, we converge the relative energies of four protonated configurations with respect to basis set and correlation level. We find that accurate relative energies require large basis sets, as well as a proper treatment of multireference and relativistic effects. We have also tested ten density functional approximations for these systems. Most of them give large errors in the relative energies. The best performing functional in this system is B3LYP, which gives mean absolute and maximum errors of only 10 and 13 kJ/mol with respect to our correlated wavefunction estimates, respectively. Our work provides benchmark results for the calibration of new approximate electronic structure methods and density functionals for these problems.