Magnetic coupling in biradicals, binuclear complexes and wide-gap insulators: a survey of ab initio wave function and density functional theory approaches

State-of-the-art computational approaches to magnetic coupling in biradical s, dinuclear complexes and wide-gap insulators are reviewed with the aim to provide a unified point of view. The most rigorous wavefunction-based meth ods provide an accurate description of magnetic coupling in all these syste ms, whereas density-functional-based methods within the broken symmetry app roach provide an alternative, yet efficient, computational tool. The use of mapping procedures permits the broken symmetry solution to be related to t he appropriate spin state. Different arguments are given to show that the n eglect of this procedure may lead to values in agreement with experiment, b ut at the cost of serious contradictions.