The close connection between significant NTO pairs and static correlation suggests that the NTOs can be used to infer electron configurations, and in particular to detect changes in electron configuration across a potential energy surface. (In fact, a more descriptive name for the NTOs might be natural electron configurations. ) Consider the electrocylic ring-opening of oxirane (C 2 H 4 O), a prototypical reaction for which the Woodward-Hoffmann rules were developed. Potential energy curves along the C-C-O bond angle of the ring are plotted in Fig. and isosurface plots of the principle NTO pair are provided at various points, for transitions to S 2 and S 3 . The reaction involves a conical intersection between these two excited states, at an angle θ CCO ≈ 62 • , and the the nonadiabatic transition that occurs there is accompanied by an abrupt switch in the qualitative nature of ψ elec 1 (r), as shown in Fig. . On the other hand, ψ hole 1 (r) remains qualitatively unchanged as the system passes through the intersection. By means of these NTOs, one may assign the diabatic character of either state: for θ CCO < 62 • , the S 2 state is n → σ * and the S 3 state is n → 3p z , whereas this character is reversed for θ CCO > 62 • . Away from any near-degeneracy between Born-Oppenheimer potential surfaces, no such abrupt change is seen in the nature of the dominant NTOs, as illustrated in Fig. .