Ground and Low‐Lying States of Cu2+—H2O. A Difficult Case for Density Functional Methods.

Abstract

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 < 2B2 < 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 2A1 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 C2v(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-intera...