The number of atoms in the unit cell is N and the occupancy factor for atom k is n k . x → → represents the point in the unit cell for which we would like to know the density of scattering matter. ρ ρ k ( x → → − − x → → k ) is the density of scattering matter from atom k at a position separated from the nuclear position x → → k by a vector x → → − − x → → k . p k ( x → → k − − x → → k 0 ) is the probability density function for displacement. x → → k 0 is the reference lattice site from which atom k may be displaced to a new position x → → k . If ρ ρ k is symmetrical enough (e.g. spherically symmetrical), x → → k 0 is simply the mean nuclear position. When considering X-ray scattering, the scattering matter density consists of electron density around the nucleus. For neutron scattering, we have δ δ -functions weighted by a scattering length b k for the respective nucleus (see Fermi pseudopotential). Note that in the above discussion, we assumed the atoms were not deformable. With this in mind, (9) may be plugged into expression (8) for the structure factor.