Born-Oppenheimer (BO) theory and its treatment for solving molecular Schr]odinger Equation
(SE), as proposed1 in 1927 and later on with Huang2 in 1954, has been the cornerstone
of our understanding of chemical processes employing quantum chemistry. The triumph
of BO treatment lies on the huge mass difference of electrons and nuclei allowing us to
separate their motions while studying molecular quantum mechanics. The approximation
allows us to study the electron dynamics which parametrically depends on the nuclear
positions. In the limiting situation of such mass differences (me MN), the BO
approximation could able to describe some of the chemical processes satisfactorily that
mainly occur at lower energy regimes of ground electronic state. However, nature exhibits
a whole range of molecular phenomena where we observe a violation of such a
'celebrated' approximation. These situations arise whenever electronic and nuclear motion
gets coupled owing to different reasons that leads to what is known as nonadiabatic
events. Simplest instances are photosynthesis, vision, charge transfer chemical reactions,
solar energy conversion and photochemical reactions, all of which involve electronically
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
is encountered quite often in nature rather than rarely.
