title: Assessment of the performance of density-functional methods for calculations on iron porphyrins and related compounds.
abstract: 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 tau-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 tau-HCTH-hyb functionals are able to provide a satisfactory description of the energetics of all the systems considered.
authors.affiliation: Department of Chemistry, P O Box 17910, Jackson State University, Jackson, Mississippi 39217, USA.
authors.forename: Meng-Sheng
authors.initials: MS
authors.lastname: Liao
authors.forename: John D
authors.initials: JD
authors.lastname: Watts
authors.forename: Ming-Ju
authors.initials: MJ
authors.lastname: Huang
mesh_terms: D007501:Iron / Q000737:chemistry*; D011166:Porphyrins / Q000737:chemistry*
chemical_list: D011166:Porphyrins; D007501:Iron