We've dropped the atomic index. a → → j belongs to the direct lattice while a → → j would belong to the reciprocal lattice. By choosing the convenient dimensionless basis a j a → → j , we guarantee that Δ Δ ξ ξ j will have units of length and describe the displacement. The tensor U in (15) is the anisotropic displacement parameter. With dimension (length) 2 , it is associated with mean square displacements. For the mean square displacement along unit vector n ^ ^ , simply take n ^ ^ T U n ^ ^ . Related schemes use the parameters β β or B rather than U (see to Trueblood et al. for a more complete discussion). Finally, we can find the relationship between the Debye–Waller factor and the anisotropic displacement parameter.