  The atomic pair distribution function (PDF) for nano-particles has to be
corrected with a shape factor, also known as form factor, in order to take size
and shape effects into consideration. For most anisotropic shapes an analytical
formulation of the shape factor is a challenge. In this paper, we present a new
method to numerically compute the shape factor using atomistic model. This
numerical method to calculate PDF for a wide range of geometrical shapes is
demonstrated. In addition, a fitting formulation, which effectively converts
the numerical shape factors for some selected geometrical shapes to the
analytical forms, is presented.
