Quantum spin correction scheme for ab initio spin-unrestricted solutions: Multiple bonds case

A quantum spin-correction scheme based on spin-correlation functions is proposed for multiple bond-breaking processes described by the unrestricted single determinant (USD) approaches, such as unrestricted Hartree-Fock (UHF) and unrestricted Kohn-Sham (UKS) density functional theory (I)FT). It is known that natural orbitals (NOs) and occupation numbers (ONs) contain full information of USD solutions. We introduce a bond-operator representation of UHF and UKS wave functions to exploit information about those NOs, enabling us not only to facilitate the calculus, but also to understand the essential feature of the USD and those quantum spin-corrected solutions. We found that usual spin-projected solutions do not work for multiple bonds in the Heisenberg model mapping scheme. We propose quantum spin-correction schemes to obtain the spin-symmetry allowed solutions from the spin-unrestricted KS-DFT solutions. We examine these schemes for a chromium dimer, which is a typical example exhibiting multiple bonds. (c) 2005 Wiley Periodicals, Inc.