Assessment and Validationof Density Functional Approximationsfor Iron Carbide and Iron Carbide Cation.

Using quantum chemical approximations to understand andpredictcomplex transition metal chemistry, such as catalytic processes andmaterials properties, is an important activity in modern computationalchemistry. High-level theory can sometimes provide high-precisionbenchmarks for systems containing transition metals, and these benchmarkscan be used to understand the reliability of less expensive quantumchemical approximations that are applicable to complex systems. Here,we studied the ionization potential energy of Fe and FeC and the bonddissociation energies of FeC and FeC+by 15 density functionalapproximations: M05, M06, M06-L, ωB97, ωB97X, ωB97X-D,τ-HCTHhyb, BLYP, B3LYP, M08-HX, M08-SO, SOGGA11, SOGGA11-X,M11, and M11-L. All of the functionals predict the correct spin stateas the ground state of neutral iron atom, but five of them predictthe wrong spin state for Fe+. In the final analysis, fourfunctionals, namely M11-L, τ-HCTHhyb, SOGGA11, and M06-L, havesmall mean unsigned errors when averaged over two bond dissociationenergies and two ionization potentials. In fact, the results showthat M11-L gives the smallest averaged mean unsigned error, i.e.,M11-L is the most reliable density functional for these iron carbidesystems among those studied. [ABSTRACT FROM AUTHOR]