Comparison of density functional and coupled cluster methods in the study of metal–ligand systems: Sc–CO2 and Cu–NO2

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Abstract
The structure, binding energies, and vibrational frequencies have been determined for the 1A1 state of the η2‐O,O coordination mode of Cu–NO2 and the 2A1 state of the η2‐O,O coordination mode of Sc–CO2. Calculations have been done using coupled cluster methods and methods based on the density functional theory. The results obtained show that all the levels of calculation lead to very similar equilibrium geometries and vibrational frequencies, while different results are obtained for the binding energy. For Sc–CO2 density functional methods overestimate the binding energy with respect to coupled cluster, while for Cu–NO2 the density functional binding energies are lower than the coupled cluster value. In both cases the inclusion of the exact Hartree–Fock exchange into the functional leads to an improvement of the density functional result. Our best estimates for the binding energies of Sc–CO2 and Cu–NO2 are 25 and 50 kcal mol−1, respectively.