Comparison between spin restricted and unrestricted post-Hartree—Fock calculations of effective exchange integrals in Ising and Heisenberg models
1993; Elsevier BV; Volume: 210; Issue: 1-3 Linguagem: Inglês
10.1016/0009-2614(93)89124-z
ISSN1873-4448
AutoresK. Yamaguchi, Mitsutaka Okumura, Wasuke Mori, Jun Maki, Kei Takada, Tomoya Noro, Kazuo Tanaka,
Tópico(s)Electron Spin Resonance Studies
ResumoSpin-restricted and unrestricted post-Hartree—Fock calculations were carried out for clusters of triplet methylene and nitroxide radicals. The UHF-based methods such as UMP and QCISD followed by approximate spin projection provide reasonable energy differences between the high-and low-spin states of these species. They are close to the corresponding values from spin-restricted multi-reference (MR) approaches such as CASSCF and second-order (SO) CI. Implications of SOCI and MRSDCI results are discussed in relation to the size inconsistency erros involved in ab initio calculations of weak interaction energies, such as the effective exchange integrals in Ising and Heisenberg models.
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