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.
Ville J. Härkönen (Mon,) studied this question.