GGA + U modeling of structural, electronic, and magnetic properties of iron porphyrin-type molecules

An ab initio computational study of various iron porphyrin-type molecules has been performed. Our ab initio calculations are based on the density functional theory (DFT) and have been conducted using the generalized gradient approximation (GGA, with PW91 & PBE versions) as well as GGA + U approach, in which an additional Hubbard- U term is added for the treatment of strong on-site 3d electron-electron interactions on Fe. We have, first, optimized the atomic distances for the porphyrin models by minimizing the total energy. Second, we benchmarked our computational approach by comparison to existing calculated results for relatively small porphyrin models obtained by the Becke-Lee-Yang-Parr (BLYP) exchange-correlation functional. We have considered several models of ligated porphyrins (Cl and NH ₃ ligated), as well as charged and neutral molecules, to study properties of the molecules as a function of oxidation state and local chemical environment of the Fe atom. We find that the GGA + U (with U ≈ 4 eV) approach provides a better description of the molecular electronic properties for five coordinated (Fe ^III) systems than the standard GGA approach, which indicates that Coulombic electron interaction effects on the Fe are important and play an essential role, particularly for the spin moment on the molecule. Also, we proceed to a larger, more realistic Fe-porphyrin model (FeP), for which we also investigate the performance of the GGA and GGA + U functionals. The character of the electronic states involved in the chemical bonding has been analyzed with the aid of energy resolved charge densities.