  We compute the ab-initio electron density beyond the Born-Oppenheimer
approximation in crystalline LiH and LiD with density functional methods. We
report significant beyond Born-Oppenheimer corrections to electron density in
the vicinity of nuclei equilibrium positions. We thus verify the breakdown of
the Born-Oppenheimer approximation, as earlier suggested on experimental
grounds. A reasonable agreement between the experimental and computational
results is found. The results indicate the existence of beyond 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.
