If the position of atoms or particles is known, then the translational order can be determined with the translational correlation function G G → → ( R → → ) as function of the distance between lattice site at place R → → and the place 0 → → , based on the two-dimensional density function ρ ρ G → → ( R → → ) = e i G → → ⋅ ⋅ [ R → → + u → → ( R → → ) ] in reciprocal space :