Performance of density functional theory methods for the treatment of metal-ligand dications

The performance of different density functional theory approaches for the treatment of MgX2+ (X=H2O, CH2O, CH3OH, NH3, CH2NH, HCN, CH3NH2, NH2OH) and CaX2+ (X=H2O, NH3) metal-ligand dications was investigated. The DFT results were compared with high-level ab initio calculations carried out at the QCISD(T)/6-311+G(3df,2p)//QCISD/6-311G* and CCSD(T)/6-311+G(3df,2p)//QCISD/6-311G* levels of theory. In general, the hybrid DFT methods yield X–Mg2+ bond distances which are too short compared with the QCISD/6-311G* optimized ones. In contrast, non-hybrid DFT approaches, such as BLYP or G96LYP, yield longer X–Mg2+ bond distances, which are in better agreement with the QCISD ones. The DFT methods investigated, with the exception of the G96LYP approach, yield Mg2+ binding energies 2.0 to 6.0 kcal/mol larger than those obtained using high-level ab initio techniques. These differences are smaller when the metal dication is Ca2+.