Now we see the overall structure factor may be represented as a weighted sum of structure factors F k ( q → → ) corresponding to each atom. Set the displacement between the location in space for which we would like to know the scattering density and the reference position for the nucleus equal to a new variable t → → = x → → − − x → → k 0 . Do the same for the displacement between the displaced and reference nuclear positions u → → = x → → k − − x → → k 0 . Substitute into (10).