The resonating broken-symmetry configuration interaction (RBS-CI) approach, in which we employ two spin-unrestricted Hartree–Fock (UHF) solutions as the basis, is applied for the ion-radical systems. The RBS-CI results are compared with those of UHF, various hybrid density functional theory (DFT), and CASSCF-DFT. The hybrid DFT leads to the spurious delocalized state as is well known, whereas the UHF to the localized state. The CASSCF-DFT results show that the excess electron localized, missing the spatial symmetry of the systems. On the other hand, the Res-HF CI results satisfy the spatial symmetry as well as the Perdew, Parr, Levy, and Balduz's relation, implying that this method is so-called many-electron self-interaction free (ME-SIF), whereas all other methods are not ME-SIF. These results suggest that the RBS-CI approach will become not only a powerful ab initio approach, but also a guideline to construct a new exchange-correlation functional of hybrid-DFT, for the ion-radical systems. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008
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