The Born-Oppenheimer approximation--that we may treat electronic and nuclear motion as occuring on different timescales--is ubiquitous in modern quantum chemistry and is crucial to our understanding and reasoning about chemical systems. In the vicinity of a conical intersection, however, the nonadibatic coupling between different electronic energy states grows rapidly, leading to a breakdown in the Born-Oppenheimer approximation. In this paper I explore conical intersections through the lens of the Born-Oppenheimer approximation and describe their relevance to isomerization reactions in photochemistry. The ethylene system is used as a model to consider more complex photochemical systems. 

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