Molecular dynamics (MD) simulations on samples of pure a-Si (composed of 216 Si-atoms) and fluorine doped a-Si (with one additional F-atom) in the temperature range 600-850K are described. The MD simulations have revealed large transient coordinate fluctuations (LTCFs) of single Si- and F-atoms in a-Si. The simulations provide the first direct MD evidence for the existence of Short-lived Large Energy Fluctuations (SLEFs) of single atoms in solids at finite temperatures. Such fluctuations of lifetime Δ τ = 10 −13−10 −12 s are predicted by the kinetic many-body SLEF theory which has been sucessfully applied to different materials including a-Si and a-Si:H. The values of Δ τ, characteristic time intervals and frequencies of spontaneous emergence of SLEFs found in the MD simulations are in good agreement with the SLEF theory predictions. Further predictions of the SLEF theory that can be verified in future MD studies are proposed.