Assessment of the performance of density‐functional methods for calculations on iron porphyrins and related compounds

The behaviors of a large number of GGA, meta‐GGA, and hybrid‐GGA density functionals in describing the spin‐state energetics of iron porphyrins and related compounds have been investigated. There is a large variation in performance between the various functionals for the calculations of the high‐spin state relative energies. Most GGA and meta‐GGA functionals are biased toward lower‐spin states and so fail to give the correct ground state for the high‐spin systems, for which the meta‐GGA functionals show more or less improvement over the GGA ones. The GGA functionals that use the OPTX correction for exchange show remarkably high performance for calculating the high‐spin state energetics, but their results for the intermediate‐spin states are somewhat questionable. A heavily parameterized GGA functional, HCTH/407, provides results which are in qualitative agreement with the experimental findings for the iron porphyrins [FeP, FeP(Cl), FeP(THF) 2 ], but its relative energies for the high‐spin states are probably somewhat too low. The high‐spin state relative energies are then even more underestimated by the corresponding meta‐GGA functional τ‐HCTH. For the hybrid‐GGA functionals, the Hartree‐Fock (HF)‐type (or exact) exchange contribution strongly stabilizes the high‐spin states, and so the performance of such functionals is largely dependent upon the amount of the HF exchange admixture in them. The B3LYP, B97, B97‐1, and τ‐HCTH‐hyb functionals are able to provide a satisfactory description of the energetics of all the systems considered. © 2006 Wiley Periodicals, Inc. J Comput Chem 27:1577–1592, 2006