Performance of density functional methods. Some difficult cases for small systems containing Cu, Ag, or Au.

Twenty-six density functional theory (DFT) methods were tested in conjunction with three different effective core potentials (ECPs) and their corresponding valence basis sets, for studying the behavior of DFT methods in small systems containing Cu, Ag, or Au where it is well-known that some functionals fail. The DFT results were compared with those obtained with post Hartree-Fock methods: second-order many-body perturbation theory (MP2), coupled cluster singles-doubles (CCSD), and coupled cluster singles-doubles with perturbative triples, CCSD(T). Calculations were carried for M3 (M = Cu, Ag, Au); M4(-), (M = Cu, Ag) and H2O-Cu. The comparison of the DFT calculated values with the Post Hartree-Fock values showed that, in general, all generalized gradient approximation (GGA) type functionals fail to describe these systems. The hybrid GGA functionals (H-GGA) showed a better behavior; however, when the Lee-Yang-Parr (LYP) exchange-correlation functional was used, wrong results were obtained. The results with the hybrid meta (HM-GGA) functionals, as in the case of H-GGAs, showed that, to obtain similar results to MP2 or CCSD(T), it is necessary to have a high Hartree-Fock exchange percentage. Spurious results obtained with the H-GGA or HM-GGA methods can be eliminated increasing the Hartree-Fock exchange percentage in the H-GGA or HM-GGA type functionals. Among the different functionals tested, the BB1K and MPWB1K functionals showed the best agreement with the MP2 and CCSD(T) results.