Ellipticity: a convenient tool to characterize electrocyclic reactions.

The ellipticity of the electron density is proposed as a convenient tool to characterize electrocyclic reactions. The study of the electron density offers several advantages since it is an experimental observable. This simple topological index has proven powerful and robust for the characterization of a wide variety of electrocyclic and pseudoelectrocyclic processes. This property is sensitive to changes in the anisotropy of the electron density in the bond-forming region and provides an insightful description of the events occurring along the reaction coordinate.