Birefringences: a challenge for both theory and experiment

A survey of some methods used by the Quantum Chemist to treat many electron systems and that might be applied to the study of exotic molecular species is presented. Emphasis is given to approaches which allow to treat static and dynamic correlation. Account is given of the results of extensive calculations of the electronic energy and structural properties of molecular fractionary charge ions, simulating species where a free quark or antiquark was captured by the atomic nucleus. The perspectives of a complete quantum chemical study of molecular exotic species, for instance the hypothetical He2 molecule, are discussed.