Non-orthogonal approaches to the study of magnetic interactions

Early theoretical studies of magnetic interactions between paramagnetic centres in solids and molecules are briefly reviewed as an introduction to the main theme of this paper: nonorthogonal CI approaches for the prediction and interpretation of magnetic couplings. In a non-orthogonal CI approach, the wavefunctions are linear combinations of configuration state functions, each of which is expressed in its own optimized orbital basis set. The NOCI approach allows for an adequate treatment of near-degeneracy correlation effects using a compact, transparent wavefunction. This facilitates straightforward analysis of the physical effects involved. A closely related method is state interaction, where the final wavefunctions are linear combinations of multi-configuration functions. Comparisons are made with the use of conventional configuration interaction and perturbation theory methods. The compound La2CuO4 is selected as an illustrative example.