Anisotropic displacement parameters are often useful for visualizing matter. From (15), we may define ellipsoids of constant probability for which γ γ = u → → T U u → → , where γ γ is some constant. Such " vibration ellipsoids " have been used to illustrate crystal structures. Alternatively, mean square displacement surfaces along n ^ ^ may be defined by ⟨ ⟨ u → → 2 ⟩ ⟩ n ^ ^ = n ^ ^ T U n ^ ^ . See the external links "Gallery of ray-traced ORTEP's", "2005 paper by Rowsell et al.", and "2009 paper by Korostelev and Noller" for more images. Anisotropic displacement parameters are also refined in programs (e.g. GSAS-II) to resolve scattering spectra during Rietveld refinement.