Cu2+ binding chalcogen-chalcogen bridges: A problematic case for DFT

In this work, we have investigated the binding of Cu2+ with the simplest chalcogen-chalcogen bridge models, i.e. H2O2, H2S2 and H2Se2. Different functionals were used to calculate binding energies and electronic properties: the simple local spin density (SVWN), some GGA (BLYP and PBE), some meta-GGA (TPSS and VSXC), different hybrid-GGA, moduling also the percentage of Hartree-Fock on the exchange contribution (B3LYP, PBE0, B98, MPW1PW91, BHandH, BHandHLYP) and two new hybrid-meta-GGA (M05 and M05-2X) functionals. Results are compared with CCSD(T) calculations and, when available, with experimental data. DFT seems to provide very different results as a function of the chosen functional and the only almost uniform message is that results can be improved by modifying the contribution of Hartree-Fock exchange. In particular, changing progressively the weight of nonlocal Hartree-Fock exchange, we found that using a value of 45% we can minimize differences with respect to CCSD(T) calculations. Finally, we noticed that adding another ligand, and thus saturating the metal coordination sphere, energies for ligand exchange reactions are less dependent to the choice of the functional.