Neumann-Wigner noncrossing rule

At this point a small digression is warranted. Both the general significance of the noncrossing rule and the opposite behaviour in photochemical reactions with respect to the corresponding thermal processes have the character of an aesthetic canon. There has been considerable resistance in the literature to the new insight that the true nature of von Neumann and Wigner s proof implies that conical intersections are everywhere, and that... 

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