Weighting of the contributing structures in terms of their contribution to the overall structure can be calculated in multiple ways, using "Ab initio" methods derived from Valence Bond theory, or else from the Natural Bond Orbitals (NBO) approaches of Weinhold [http://www.chem.wisc.edu/~nbo5 NBO5] , or finally from empirical calculations based on the Hückel method. A Hückel method-based software for teaching resonance is available on the [http://www.hulis.free.fr HuLiS] Web site.