COMPARISON BETWEEN SPIN RESTRICTED AND UNRESTRICTED POST-HARTREE-FOCKCALCULATIONS OF EFFECTIVE EXCHANGE INTEGRALS IN ISING AND HEISENBERG MODELS

Spin-restricted and unrestricted post-Hartree-Fock calculations were c arried out for clusters of triplet methylene and nitroxide radicals. T he UHF-based methods such as UMP and QCISD followed by approximate spi n projection provide reasonable energy differences between the high- a nd low-spin states of these species. They are close to the correspondi ng values from spin-restricted multi-reference (MR) approaches such as CASSCF and second-order (SO) Cl. Implications of SOCI and MRSDCI resu lts are discussed in relation to the size inconsistency errors involve d in ab initio calculations of weak interaction energies, such as the effective exchange integrals in Ising and Heisenberg models.