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 C-2upsilon symmetry and that the ground electronic state is a (2)A(1) state. At this level of theory the relative order of the electronic states is (2)A(1) < B-2(1) < B-2(2) < (2)A(2). 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 similar to20-25%, the B-2(1) state becomes more stable than the (2)A(1) one. Moreover, with these functionals a C-s((2)A') structure is found to be the ground-state structure of Cu2+-H2O. This is attributed to the fact that, for C-2upsilon(B-2(1)) and C-s((2)A'), 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.