On the breakdown of the Born-Oppenheimer approximation in LiH and LiD

We compute the ab-initio electron density beyond the strict Born-Oppenheimer approximation in crystalline LiH and LiD with density functional methods. By taking into account the quantum mechanical nature of the nuclei, an aspect absent in the strict Born-Oppenheimer approximation, we find significant corrections to electron density in the vicinity of nuclei equilibrium positions. We compare our results with earlier experimental findings that have suggested a breakdown of the Born-Oppenheimer approximation in these solids and obtain improved agreement between experiment and theory when quantum nuclear effects are included. A notable temperature dependence of electron density is found. The results indicate the existence of beyond strict Born-Oppenheimer effects in solids at normal pressures and suggest that such effects can be significant also in materials containing light elements other than hydrogen.