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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2004-07, Vol.108 (28), p.6072-6078
• Main Authors: Poater, J, Solà, M, Rimola, A, Rodríguez-Santiago, L, Sodupe, M
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp0487657

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 2 v 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 C s (2A‘) structure is found to be the ground-state structure of Cu2+−H2O. This is attributed to the fact that, for C 2 v (2B1) and C s (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.