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The Born–Oppenheimer (BO) approximation1 allows the separation of electronic and nuclear degrees of freedom and the definition of individual potential energy surfaces (PESs) for different electronic states. It provides a valid description of many chemical processes and is thus invoked in most quantum chemical calculations. However, there are also a number of phenomena that cannot be described within this framework. When two electronic states of the same multiplicity become degenerate, there will generally be a pronounced interstate coupling that leads to a strong electronic mixing and a breakdown of the BO approximation. In the region of such conical intersections, the strong coupling of the electronic and nuclear motion induces so-called nonadiabatic transitions between different electronic states which are at the heart of photochemistry, internal conversion, fluorescence quenching, and nonradiative energy dissipation processes. 2–9In recent years, many theoretical tools have been developed to study conical intersections and nonadiabatic phenomena. The location 10–16 of minimum-energy conical intersections4, and of conical intersection seams17–20 can provide information on the topology of the relevant PESs and help to find geometrical configurations that are crucial for the dynamics. Excited-state reaction channels can be found by constructing minimum-energy reaction paths connecting the Franck–Condon region with different conical intersections, thus identifying favorable pathways and the associate energy barriers. A more complete characterization of nonadiabatic processes requires the direct simulation of nonadiabatic …
World Scientific
Publication date: 
1 Jan 2011

Eduardo Fabiano, Zhenggang Lan, You Lu, Thiel Walter

Biblio References: 
Conical Intersections: Theory, Computation and Experiment