Reprinted with permission from the Journal of Physical Chemistry A. Copyright 2009 American Chemical Society.
Abstract: The performance of 44 density functionals used in conjunction with the correlation consistent basis sets (ccpVnZ where n = T and Q) has been assessed for the gas-phase enthalpies of formation at 298.15 K of 3d transition metal (TM) containing systems. Nineteen molecules were examined: ScS, VO, VO₂, Cr(CO)₆, MnS, MnCl₂, Mn(CO)₅Cl, FeCl₃, Fe(CO)₅, CoH(CO)₄, NiCl₂, Ni(CO)₄, CuH, CuF, CuCl, ZnH, ZnO, ZnCl, and Zn(CH₃)₂. Of the functionals examined, the functionals that resulted in the smallest mean absolute deviation (MAD, in parentheses, kcal molˉ¹) from experiment were B97-1(6.9), PBE1KCIS (8.1), TPSS1KCIS (9.6), B97-2(9.7), and B98(10.7). All five of these functionals include some degree of Hartree-Fock (HF) exchange. The impact of increasing the basis set from cc-pVTZ to cc-pVQZ was found to be slight for the generalized gradient approximation (GGA) and meta-GGA (MGGA) functionals studied, indicating basis set saturation at the triple-ζ level. By contrast, for most of the generalized gradient exchange (GGE), hybrid GGA (HGGA), and hybrid meta-GGA (HMGGA) functionals considered, improvements in the average MAD of 2-3 kcal molˉ¹ were seen upon progressing to a quadruple-ζ level basis set. Overall, it was found that the functionals that include Hartree-Fock exchange performed best overall, but those with greater than 40% HF exchange exhibit significantly poor performance for the prediction of enthalpies of formation for 3d TM complexes. Carbonyl-containing complexes, a mainstay in organometallic TM chemistry, are demonstrated to be exceedingly difficult to describe accurately with all but 2 of the 44 functionals considered. The most accurate functional, for both CO-containing and CO-free compounds, is B97-1/cc-pVQZ, which is shown to be capable of yielding results within 1 kcal molˉ¹ of high-level ab initio composite methodologies.
