Instability In Chemical Bonds: Uno Cascc, Resonating Ucc And Approximately Projected Ucc Methods To Quasi-Degenerate Electronic Systems

We review several versions of coupled-cluster (CC) methods based on spin-unrestricted Hartree–Fock (UHF) approximation. A multireference (MR) CC theory by the use of UHF natural orbitals (UNO), UNO CASCC, is revisited to elucidate scope and limitation of UHF coupled-cluster (CC) methods and two types of quantum corrections for UHF CC (UCC) solutions: first one is the approximate spin-projected UCC scheme (AP UCC) based on Heisenberg model Hamiltonian, and second one is the recently developed resonating UCC configuration interaction (Res-UCC CI) approach. In this article, we examine these CC-based approaches for quasi-degenerate electronic systems such as dissociation of covalent bonds. The computational results are discussed in comparison with those of the MR-CC theory. Our methods have been applied to exchange-coupled systems and ion-radical systems, which have been accepted current interests.