Intersections in Molecular Systems

Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122 

The study of molecular systems using quantum mechanics is based on the Born-Oppenheimer approximation. This approximation relies on the fact that the electrons, because of their smaller mass, move much faster than the heavier nuclei, so they follow the motion of the nuclei adiabatically, whereas the latter move on the average potential of the former. The Born-Oppenheimer approximation is sufficient to describe most chemical processes. In fact, our notion of molecular structure is based on the Born-Oppenheimer approximation, because the molecular structure is formed by nuclei being placed in fixed positions. There are, however, essential nonadiabatic processes in nature that cannot be described within this approximation. Nonadiabatic processes are ubiquitous in photophysics and photochemistry, and they govern such important phenomena as photosynthesis, vision, and charge-transfer reactions. 

Based on the Born-Oppenheimer approximation, the behavior of molecules is described by the dynamics of the nuclei moving along a single potential energy surface (PES) generated by the electrons. Nonadiabatic phenomena occur when at least two potential energy surfaces approach each other and the coupling between them becomes important. The traditional 

Reviews in Computational Chemistry, Volume 23 edited by Kenny B. Lipkowitz and Thomas R. Cundari Copyright 2007 Wiley-VCH, John Wiley 6c Sons, Inc. 

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Matsika S (2007) Conical intersections in molecular systems. In Lipkowitz KB and Cundari TR (eds.) Reviews in computational chemistry, vol. 23, Wiley-VCH, New Jersey, pp 83-124... 

Spiridoula Matsika, Conical Intersections in Molecular Systems. 

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