Quantities used in condensed-matter physics that are ensemble averages of products of such quantities as density at different points in space. Each particle affects the behaviour of its neighbouring particles, so creating correlations, the range of which is at least as long as the range of the intermolecular forces. Thus, correlation functions contain a great deal of information about systems in condensed-matter physics. Correlation functions can be measured in considerable detail, frequently by experiments involving scattering of such particles as neutrons or of electromagnetic radiation (e.g. light or X-rays) and can be calculated theoretically using statistical mechanics.