The Born-Oppenheimer (adiabatic) approximation completely separately electronic & nuclear degrees of motion, and is adequate for most ground state problems. However, it breaks down when the energy gap between multiple electronic states become small compared to the vibrational energy scale. The resulting coupling between electronic and nuclear motion permits nonradiative population transfer between electronic states. This is common in excited state dynamics, where internal conversion usually occurs through “conical intersections” between Born-Oppenheimer potential energy surfaces.
Simulation of nonadiabatic dynamics thus important for many areas of chemistry and requires quantum treatment of both nuclei and electrons. The most common approach is to add an approximate description of nonadiabaticity (surface hopping or ab initio multiple spawning) to molecular dynamics. However, it is also possible to map problems to simplified model potentials (like the spin-boson model) where higher level quantum treatment for nuclei can be carried out. My work in this area had explored both aspects. Specifically, I have explored the role of nonadiabatic effects in photochemistry and developed new methods for simulating the electronic state population dynamics of the spin-boson model.
Diptarka Hait 