Oral: Exact factorization of the many-body Green's function theory.

The Born-Oppenheimer (BO) approximation is the cornerstone of condensed matter physics and essentially all ab initio computations on crystalline solids are based on it. At the same time, experiments imply that there are some systems in which the BO approximation may be compromised. As the beyond-BO wave function approach is computationally too demanding in systems like crystals, alternative approaches are needed. An alternative beyond-BO approach is the method of many-body Green's functions. Such a theory was introduced as early as the 1960s, but it has been computationally too expensive to be implemented in practice. To make the beyond-BO Green's function theory computationally more feasible, we combined the many-body Green's function approach and the exact factorization of the wave function approach. This exact approach allows a systematic scheme for deriving beyond-BO approximations. With the results obtained from this approach, we have recently, for the first time, computed ab-initio electron densities beyond the strict BO approximation for experimentally known hydrogen rich solids. Our results show a breakdown of the strict BO approximation as suggested by the earlier experimental work. Consequently, our results imply that in order to have an accurate description of the electronic structure of these systems, the quantum mechanical nature of the nuclei have to be taken into account. These effects are ignored in the current state of the art methods for crystals assuming the strict BO approximation.