Choice of functional for iron porphyrin density functional theory studies: Geometry, spin-state, and binding energy analysis

Abstract

Simultaneously predicting the geometries, spin states and binding energies of iron porphyrins using DFT methods is challenging, as one method and basis set could lead to the accurate prediction of one aspect but imprecisions on the others. In this work, six functionals and three basis sets were tested based on their description of the geometries, spin states and binding energies of an heme model. An iron porphyrin ring with an axial imidazole (FePIm) was used as the model molecule. Optimized geometries of FePIm and dioxygen-coordinated FePIm-O2 were obtained in their singlet, triplet, and quintet spin states. Energy calculations with zero-point vibrational energy (ZPVE) corrections were performed on the optimized geometries, at the same level of theory used on the geometry optimization. For the oxygen-bonded FePIm-O2, the singlet spin states were calculated to be closed-shell for all the methods, even if an unrestricted wavefunction and broken spin symmetry were imposed on the system. Consequently, the open-shell singlets were obtained using the wavefunction of their corresponding triplet states as the initial guess for the geometry optimizations and energy calculations. The functional B97D with the basis sets A (def2-TZVP on Fe, def2-SVP on other atoms) and C (6–311+G(2df) on Fe atom; 6–311+G(d) on N and O atoms; 6–31G on C and H atoms) was able to produce an accurate geometry, the correct spin states for FePIm (Quintet) and FePIm-O2 (open-shell singlet), and a binding energies of 13.51–13.07 kcal·mol−1, in good agreement to the experimental values. The basis set B resulted in good FePIm geometrical parameters with all the studied functionals, but led to significant errors in the FePIm-O2 geometries and binding energies. For the study of FePIm and similar molecules were the change of spin play a key role, the selection of constraints and initial guess are very important, as the methods could yield multiplicites which do not agree with experimental observations. This study provides valuable information for the selection of a suitable method for DFT studies on small iron porphyrins and their interaction with oxygen, based on the required accuracy for geometry, spin, or binding energies.