where q, u are the magnitudes (or absolute values) of the vectors q, u respectively, and ⟨ ⟨ u 2 ⟩ ⟩ is the mean squared displacement. In crystallographic publications, values of U are often given where U = ⟨ ⟨ u 2 ⟩ ⟩ . Note that if the incident wave has wavelength λ λ , and it is elastically scattered by an angle of 2 θ θ , then