Pair distribution function
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The atomic pair distribution function (PDF) describes the density of interatomic distances in a material. Of special practical importance is the radial pair distribution function, which is independent of orientation. It is a major descriptor for the atomic structure of amorphic materials (glasses, polymers) and liquids. The radial PDF can be calculated directly from physical measurements like light scattering or x-ray powder diffraction.
In Statistical Mechanics the PDF is given by the expression:
