Applications of effective core potentials and density functional theory to the spin states of iron porphyrin

We investigated the performance of the B3LYP density functional in combination with ab initio effective core potentials (ECPs) that are derived from either Hartree-Fock or Dirac-Fock calculations. The transferability of ab initio ECPs is assessed on the basis of comparison with all-electron density functional calculations. For iron(II) porphyrin in particular, our assessment focused on the relative energetic ordering of five low-lying spin states, 1A1G, 3A1G, 3B2G, 5A2G, and 5B1G, and their properties, including optimized structures, charge distribution, spin density, and vibrational frequencies. Our results show that core electron correlation and core-valence electron correlation do not have significant effects on the relative energetics of the spin states of iron porphyrin. Our calculations suggest that effects of replacing the core electrons with ECPs are less significant than the choice of basis functions. We conclude that ab initio ECPs such as LANL2, RCEP, and MEFIT-R may be combined with the B3LYP density functional theory to provide consistent and accurate results.