Ab initio computations of effective exchange integrals for H-H, H-He-H and Mn2O2 complex:: comparison of broken-symmetry approaches

Ab initio calculations of effective exchange interactions between spins are performed for H-H, H-He-H and a simplified model of binuclear manganese oxide, Mn2O2, by using the spin-unrestricted Hartree-Fock (UHF), spin-polarized density functional (DFT) and UHF + DFT hybrid methods. The scopes and limitations of these broken-symmetry approaches are discussed in relation to several computational schemes of effective exchange integrals (J(ab)). The natural orbitals (UNO or DNO) of the UHF, DFT and hybrid DFT solutions for magnetic clusters are used for interpretation of the superexchange interactions in Mn2O2 complexes. (C) 2000 Elsevier Science B.V. All rights reserved.