Comparison between spin restricted and unrestricted post-Hartree—Fock calculations of effective exchange integrals in Ising and Heisenberg models
✍ Scribed by K. Yamaguchi; M. Okumura; W. Mori; J. Maki; K. Takada; T. Noro; K. Tanaka
- Publisher
- Elsevier Science
- Year
- 1993
- Tongue
- English
- Weight
- 797 KB
- Volume
- 210
- Category
- Article
- ISSN
- 0009-2614
No coin nor oath required. For personal study only.
✦ Synopsis
Spin-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 errors involved in ab initio calculations of weak interaction energies, such as the effective exchange integrals in Ising and Heisenberg models.