title: Balancing Exchange Mixing in Density-Functional Approximations for Iron Porphyrin.
abstract: Predicting the correct ground-state multiplicity for iron(II) porphyrin, a high-spin quintet, remains a significant challenge for electronic-structure methods, including commonly employed density functionals. An even greater challenge for these methods is correctly predicting favorable binding of O2 to iron(II) porphyrin, due to the open-shell singlet character of the adduct. In this work, the performance of a modest set of contemporary density-functional approximations is assessed and the results interpreted using Bader delocalization indices. It is found that inclusion of greater proportions of Hartree-Fock exchange, in hybrid or range-separated hybrid functionals, has opposing effects; it improves the ability of the functional to identify the ground state but is detrimental to predicting favorable dioxygen binding. Because of the uncomplementary nature of these properties, accurate prediction of both the relative spin-state energies and the O2 binding enthalpy eludes conventional density-functional approximations.
authors.affiliation: Department of Chemistry, Dalhousie University , 6274 Coburg Road, Halifax, Nova Scotia Canada , B3H 4R2.
authors.forename: Victoria E J
authors.initials: VE
authors.lastname: Berryman
authors.affiliation: Department of Chemistry, Dalhousie University , 6274 Coburg Road, Halifax, Nova Scotia Canada , B3H 4R2.
authors.forename: Russell J
authors.initials: RJ
authors.lastname: Boyd
authors.affiliation: Department of Chemistry, Dalhousie University , 6274 Coburg Road, Halifax, Nova Scotia Canada , B3H 4R2.
authors.forename: Erin R
authors.initials: ER
authors.lastname: Johnson
mesh_terms: D007501:Iron / Q000737:chemistry*; D008665:Metalloporphyrins / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry; D011789:Quantum Theory; D013816:Thermodynamics
chemical_list: D008665:Metalloporphyrins; D007501:Iron; D010100:Oxygen