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Resonating broken-symmetry approach to biradicals and polyradicals



We present a resonating broken-symmetry (RBS) approach for strongly correlated electron systems. This is a nonorthogonal configuration interaction (CI) using symmetry-broken Hartree–Fock (HF) references. We have applied it to the hydrogen molecule, linear hydrogen chain model systems, and to the fluorine molecule. As is known, spin contamination of biradical system with S = 1/2 can be removed by taking the resonance of two spin-unrestricted HF (UHF) configurations. In contrast, more configurations were required to do spin projections perfectly in polyradical systems. The implications of the computational results are discussed in relation to the applicability of the RBS molecular orbital (MO) approach to more complex systems, such as metal clusters. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006