Ab initio calculations of the electronic structure and magnetism of iron porphyrin-type molecules: a benchmarking study

The iron porphyrin molecule is one of the most important biomolecules. In spite of its importance to life science, on a microscopic scale its electronic properties are not yet well-understood. In order to achieve such understanding we have performed an ab initio computational study of various molecular models for the iron porphyrin molecule. Our ab initio electronic structure calculations are based on the density functional theory (DFT) and have been conducted using both the Generalised Gradient Approximation (GGA) and the GGA+U approach, in which an additional Hubbard-U term is added for the treatment of on-site electron-electron correlations. In our investigations we have, first, optimised the molecular structures by computing the minimal-energy atomic distances, and second, benchmarked our computational approach by comparison to existing calculated results obtained by quantum-chemical methods. We have considered several models of ligated porphyrin (Cl and NH3 ligated), as well as charged and non-charged molecules. In this way, the changes in the electronic, structural, and magnetic properties of the iron atom have been investigated as a function of the oxidation state and local environment of the iron atom. Our results for some of the model molecules reproduce the earlier quantum-chemical calculations done by Johansson and Sundholm [J. Chem. Phys. 120 (2003) 3229]. We find that the GGA+U approach provides a better description of the molecular electronic properties, which indicates that electron correlation effects on the iron are important and play an essential role, particularly for the spin moment on the iron atom. Also, we proceed beyond the relatively small molecular models to a larger, more realistic porphyrin molecule, for which we also find that the GGA+U results are in better agreement with experiments.