I am not sure quite what you mean by this, but I think you refer to the characteristic function that damps the PDF with increasing-r due to finite particle sizes. The form of this depends on what shape you select (in diffpy-cmi you can do spherical particle, spheroidal, core-shell, log-normal distributions of spheres among others). The most commonly used one is for the sphere. I think you can find the functional form for that one in the underneath the bragg peaks book. There are a number of other great papers that describe these characteristic functions for the other shapes.