title: How Much Can Density Functional Approximations (DFA) Fail? The Extreme Case of the FeO4 Species.
abstract: A thorough theoretical study of the relative energies of various molecular Fe·4O isomers with different oxidation states of both Fe and O atoms is presented, comparing simple Hartree-Fock through many Kohn-Sham approximations up to extended coupled cluster and DMRG multiconfiguration benchmark methods. The ground state of Fe·4O is a singlet, hexavalent iron(VI) complex (1)C2v-[Fe(VI)O2](2+)(O2)(2-), with isomers of oxidation states Fe(II), Fe(III), Fe(IV), Fe(V), and Fe(VIII) all lying slightly higher within the range of 1 eV. The disputed existence of oxidation state Fe(VIII) is discussed for isolated FeO4 molecules. Density functional theory (DFT) at various DF approximation (DFA) levels of local and gradient approaches, Hartree-Fock exchange and meta hybrids, range dependent, DFT-D and DFT+U models do not perform better for the relative stabilities of the geometric and electronic Fe·4O isomers than within 1-5 eV. The Fe·4O isomeric species are an excellent testing and validation ground for the development of density functional and wave function methods for strongly correlated multireference states, which do not seem to always follow chemical intuition.
authors.affiliation: Department of Chemistry and Key Laboratory of Organic Optoelectronics & Molecular Engineering of the Ministry of Education, Tsinghua University , Beijing 100084, China.
authors.forename: Wei
authors.initials: W
authors.lastname: Huang
authors.affiliation: Department of Chemistry and Key Laboratory of Organic Optoelectronics & Molecular Engineering of the Ministry of Education, Tsinghua University , Beijing 100084, China.
authors.forename: Deng-Hui
authors.initials: DH
authors.lastname: Xing
authors.affiliation: Department of Chemistry and Key Laboratory of Organic Optoelectronics & Molecular Engineering of the Ministry of Education, Tsinghua University , Beijing 100084, China.
authors.forename: Jun-Bo
authors.initials: JB
authors.lastname: Lu
authors.affiliation: Department of Chemistry and Key Laboratory of Organic Optoelectronics & Molecular Engineering of the Ministry of Education, Tsinghua University , Beijing 100084, China.
authors.forename: Bo
authors.initials: B
authors.lastname: Long
authors.affiliation: Department of Chemistry and Key Laboratory of Organic Optoelectronics & Molecular Engineering of the Ministry of Education, Tsinghua University , Beijing 100084, China.
authors.forename: W H Eugen
authors.initials: WH
authors.lastname: Schwarz
authors.affiliation: Department of Chemistry and Key Laboratory of Organic Optoelectronics & Molecular Engineering of the Ministry of Education, Tsinghua University , Beijing 100084, China.
authors.forename: Jun
authors.initials: J
authors.lastname: Li