Ground and low-lying states of Cu²⁺-H₂O. A difficult case for density functional methods

The ground and low-lying states of Cu2+-H2O have been studied using different density functional and post-Hartree-Fock methods. CCSD(T) results indicate that Cu2+-H2O has C2v symmetry and that the ground electronic state is a 2A1 state. At this level of theory the relative order of the electronic states is 2A1 < 2B1 < 2B 2 < 2A2. However, density functional results show that the relative stabilities of these states vary depending on the degree of mixing of exact Hartree-Fock (HF) and density functional (DF) exchange. For pure generalized gradient approximation (GGA) functionals and also for hybrid functionals with percentages of HF mixing up to ∼20-25%, the 2B1 state becomes more stable than the 2A 1 one. Moreover, with these functionals a Cs( 2A′) structure is found to be the ground-state structure of Cu2+-H2O. This is attributed to the fact that, for C 2v(2B1) and Cs(2A′) , GGA functionals provide a delocalized picture of the electron hole, which is overstabilized due to a bad cancellation of the self-interaction part by the exchange-correlation functional. Among the different functionals tested, the one that provides better results compared to CCSD(T) is the BHLYP one.