Definition of corresponding orbitals and the diradical character in broken symmetry DFT calculations on spin coupled systems

The broken symmetry (BS) concept is an extremely useful tool for the prediction of exchange coupling constants in molecules with interacting paramagnetic centers. An analysis of the BS wavefunctions is presented and the relationship between the overlap of magnetic orbitals and the exchange coupling is stressed. The corresponding orbital transformation is introduced as a useful tool in order to determine the non-orthogonal ‘valence bond’-like magnetic orbital pairs in many electron systems.