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

We compute theab-initioelectron density beyond the strict Born-Oppenheimer (BO) 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 BO 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 BO 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 BO effects in solids at normal pressures and suggest that such effects can be significant also in materials containing light elements other than hydrogen.