Rubrene (5,6,11,12-tetraphenyltetracene) has recently attracted attention for applications in organic field effect transistors, due to its high carrier mobility at room temperature. This mobility decreases dramatically below 175 K and differential scanning calorimetry reveal a transition at this temperature. However, an x-ray study of the structure of rubrene between 100 and 293K has shown no changes in the crystallographic symmetry. Moreover, a band structure calculation demonstrated that a major low-frequency vibrational mode at 24 cm-1 is involved in the relaxation energy of the molecule and strongly contributes to the hopping process at high temperature. In this work we propose to study the evolution of the Anisotropic Displacement Parameters in the crystal structure of rubrene in the temperature range from 5 to 300 K, in order to elucidate the molecular motion of the molecule in the crystal and derive the low frequency molecular deformations that can be important for the high mobility above 175 K.