Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In statistical mechanics, a radial distribution function (RDF), g(r), describes how the atomic density varies as a function of the distance from one particular atom. More precisely, if there is an atom at the origin 0, and if n = N/V is the average number density, then the local density at distance r from O is ng(r). Given a potential energy function, the radial distribution function can be found either via computer simulation methods like the Monte Carlo method, or via the Ornstein-Zernike equation, using approximative closure relations like the Perckus-Yevick approximation or the Hypernetted Chain Theory.