Resonating broken symmetry CI approach for ion‐radical systems: Comparison with UHF, hybrid‐DFT, and CASSCF‐DFT

Abstract

ABSTRACT: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.

Theory of chemical bonds in metalloenzymes XIII: Singlet and triplet diradical mechanisms of hydroxylations with iron‐oxo species and P450 are revisited

“…This entails more or less the spin polarization of the Fe(IV)O (σ) 2 ‐bond, generating a reactive (namely electron‐accepting) σ*‐like BS LUMO for the up‐spin at the O‐site 23, 70. Thus, the metal‐oxene core in the weak ligand field exhibits the radical reactivity as discussed previously 23–27, 67. The 2p z ‐orbital‐like LUMO of the O‐site becomes a reactive radical site.…”

“…In this series of articles, electronic structures of the species have been studied on the basis of MO concepts such as the instability of closed‐shell MO via electron correlation and resulting BS MOs, together with the resonating BS (RBS) MO 62–64 instead of resonating VB (RVB). The multireference (MR) approaches such as MR DFT are also used for further quantitative MO descriptions of the species 65–67. Isoelectronic or isolobal analogy among O, O 2 , and Fe(IV)O based on these generalized MO concepts has been one of our basic ideas for theoretical understanding of chemical reactivity of molecular oxygen and the Fe(IV)O species in heme‐ and nonheme complexes 23–27 because a lot of experimental 1–4 and theoretical 24 studies on oxygenations with O and O 2 have been performed even at that time (near 1980) and such results are useful for comparative purpose.…”

“…The electronic state of CpdI is thus regarded as a typical system with the QDSs 68, leading to the QDS model for hydroxylation. Therefore, the multistate multiconfiguration approach is inevitable for quantitative description of CpdI 65–68.…”

Resonating coupled‐cluster CI approach to ion‐radical systems: Comparison with the unrestricted coupled‐cluster approach

“…(17) can be estimated using cofactors. After estimating the overlap integrals and transfer integrals this way, we can obtain the Res-CI solution by the manner similar to Res-HF CI [32].…”

“…In the similar manner, the virtual orbitals are transformed by U Ra i $ 0 for any pair of (a, p) for several parallel-stacking configurations of ethylene dimmer cation [32]. The problem of this treatment is that a coupled-cluster code based on noncanonical orbitals is needed, which is, to our knowledge, nonexisting.…”