Mean-square relative displacement in EXAFS (${\sigma }^2=⟨[({\mathbf{u}}_j-{\mathbf{u}}_0)\cdot \hat{\mathbf{R}}{]}^2⟩$)

The bond-direction variance of relative atomic displacement. A Gaussian distance distribution damps the coherent round-trip phase by $e^{-2k^2{\sigma }^2}$. Correlated atomic motion and static bond disorder contribute, so this is not generally a single atom's mean-square displacement about its lattice site.