Beyond born oppenheimer theories on molecular processes

<p>Born-Oppenheimer (BO) theory and its treatment for solving molecular Schr��odinger Equation</p><p>(SE) as proposed1 in 1927 and later on with Huang2 in 1954 has been the cornerstone</p><p>of our understanding of chemical processes employing quantum chemistry. The triumph</p><p>of BO treatment lies on the huge mass difference of electrons and nuclei allowing us to</p><p>separate their motions while studying molecular quantum mechanics. The approximation</p><p>allows us to study the electron dynamics which parametrically depends on the nuclear</p><p>positions. In the limiting situation of such mass differences (me MN) the BO</p><p>approximation could able to describe some of the chemical processes satisfactorily that</p><p>mainly occur at lower energy regimes of ground electronic state. However nature exhibits</p><p>a whole range of molecular phenomena where we observe a violation of such a</p><p>'celebrated' approximation. These situations arise whenever electronic and nuclear motion</p><p>gets coupled owing to different reasons that leads to what is known as nonadiabatic</p><p>events. Simplest instances are photosynthesis vision charge transfer chemical reactions</p><p>solar energy conversion and photochemical reactions all of which involve electronically</p><p>excited states and thus cannot be fully accounted for if considered solely from a BO per-spective. Owing to such range of nonadiabatic phenomena failure of BO approximation</p><p>is encountered quite often in nature rather than rarely.</p>