• ion radical;
  • resonating configuration interaction;
  • coupled-cluster methods


We implemented the resonating configuration interaction (Res-CI) approach with spin-unrestricted coupled-cluster (UCC) solutions as basis for ion-radical systems. In previous studies (Nishihara et al., Int J Quantum Chem 2008, 108, 2966; Nishihara et al., J Phys: Condens Matt 2009, 21, 064227), we showed that it is possible to satisfy an important condition proposed by Perdew et al. using Res-CI based on two complementary spin-unrestricted Hartree–Fock solutions. In this study, we extended the method to Res-CI calculations based on UCC solutions (Res-CC CI) for simple ion-radical systems, which is to our knowledge, a new type of ab initio method. We examined the asymmetric effects on whether the hole (or the excess electron) localizes at one site or delocalizes over two sites. The computational results of Res-CC CI are compared with those of UCC, and the implications are discussed. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009