The radial distribution function (RDF), also known as the pair distribution function (PDF), is a statistical measure used primarily in the fields of chemistry, physics, and materials science to describe how density varies as a function of distance from a reference particle within a system of particles. It provides insight into the structural properties of a material, particularly in liquids and gases but also in solids.
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In a homogeneous isotropic one-component fluid, is the conditional number density at separation from a tagged particle divided by the bulk number density . Thus the displayed shell contains the mean number of neighbours. For an uncorrelated ideal gas away from the tagged particle; repulsive cores suppress small separations, while liquid coordination shells can produce damped oscillations approaching one at large distance. This is a pair correlation, not a normalized one-particle radial probability density. The static structure factor measures its Fourier-space counterpart.