Optimization of Spin-Unrestricted Density Functional Theory for Redox Properties of Rubredoxin Redox Site Analogues

Quantum chemical calculations of metal clusters in proteins for redox studies require both computational feasibility as well as accuracies of at least similar to 50 mV for redox energies but only similar to 0.05 angstrom for bond lengths. Thus, optimization of spin-unrestricted density functional theory (DFT) methods, especially the hybrid generalized gradient approximation functionals, for energies while maintaining good geometries is essential. Here, different DFT functionals with effective core potential (ECP) and full core basis sets for [Fe(SCH3)(4)](2-/1-10) and [Fe(SCH3)(3)](1-/0), which are analogues of the iron-sulfur protein rubredoxin, are investigated in comparison to experiment as well as other more computationally intensive electron correlation methods. In particular, redox energies are calibrated against gas-phase photoelectron spectroscopy data so no approximations for the environment are needed. B3LYP gives the best balance of accuracy in energy and geometry as compared to B97gga1 and BHandH and is better for energies than Moller-Plesset perturbation theory series (MP2, MP3, MP4SDQ) and comparable to coupled cluster [CCSD, CCSD(T)] methods. Of the full core basis sets tested, the 6-31G** basis sets give good geometries, and addition of diffuse functions to only the sulfur significantly improves the energies. Moreover, a basis set with an ECP on only the iron gives less accurate but still reasonable geometries and energies.