Abstract: The principles of the separation ofπ-system in SCFMOmethods arc described.The π-resonance integral contribution to energy is proposed as a measure of π-bonding.It is indicated by the π-rcsonance integral contributions and other data of π-system that the C-Cbond in ethane is not a pure single bond but has a small amount of π-bonding The data showing the resemblance of ethane and propylene to butadiene are presented.The details of the algorithm for separating π-system arc presented.