Abstract: The membership function(μZ) in fuzzy mathematics has been introduced into molecular point group, and fuzzy point group may be constructed to describe the kind of molecular imperfect symmetry. Furthermore, several concepts such as fuzzy irreducible representation and fuzzy generalized parity for molecular orbital with fuzzy symmetry have been put forward. Exploring the fuzzy symmetry of prototype linear, planar and non-planar molecules, there is a new land of theoretical chemistry before us. Based on this, we have investigated the fuzzy symmetry of simple chemical reactions, revealing the quasi-quantitative characteristics of Woodward-Hoffmann rule.

Key words: Fuzzy symmetry, Molecule, Molecular orbital, Membership function, 　Woodward-Hoffmann rule