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

We compute the ab-initio electron density beyond the crude 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 crude Born-Oppenheimer approximation, we report significant corrections to electron density in the vicinity of nuclei equilibrium positions. We compare our results to earlier experimental findings that have suggested Born-Oppenheimer approximation breakdown in these solids. We find an improved agreement between the experimental and computational results when the quantum nuclear effects are taken into account. A notable temperature dependence of electron density is found. The results indicate the existence of beyond crude Born-Oppenheimer effects in solids at normal pressures and suggest that these effects can be significant also in solids containing elements other than hydrogen as well.