Subvalence Orbitals and Diffuse Functions in Accurate Electronic Structure Calculations: Insights from Nitrogenase Fe–S Dimer Clusters

This study investigates the effect of including diffuse functions and additional subvalence basis functions in benchmark electronic structure calculations on metal-ligand complexes. To conduct our analysis, we performed unrestricted Hartree-Fock (UHF) and unrestricted second-order Møller-Plesset perturbation theory (UMP2) calculations with varying basis sets on the four Fe-S dimer clusters studied by Zhai et al. (Zhai, H.; Lee, S.; Cui, Z.; Cao, L.; Ryde, U.; Chan, G. K. Multireference Protonation Energetics of a Dimeric Model of Nitrogenase Iron-Sulfur Clusters. <i>J. Phys. Chem. A</i> 2023, 127, 9974-9984, 10.1021/acs.jpca.3c06142), which are simplified models of protonated nitrogenase enzymes. We found that, while the diffuse functions and additional subvalence basis functions have minimal influence on the UHF relative energies between the four dimers and the diffuse functions have minimal effects on the UMP2 relative energies, the additional subvalence basis functions significantly affect the UMP2 relative energies. Neglecting the additional subvalence functions leads to systematic underestimations of the energy differences between the four dimers by up to 2.7 kcal/mol. We propose a revised composite method incorporating these basis functions to provide more accurate benchmark values for the four dimer clusters.