## Quick Reference

A theory relating quantities in equilibrium and nonequilibrium statistical mechanics and microscopic and macroscopic quantities. The fluctuation-dissipation theorem was first derived for electrical circuits with noise in 1928 by H. Nyquist; a general theorem in statistical mechanics was derived by H. B. Callen and T. A. Welton in 1951. The underlying principle of the fluctuation-dissipation theorem is that a nonequilibrium state may have been reached either as a result of a random fluctuation or an external force (such as an electric or magnetic field) and that the evolution towards equilibrium is the same in both cases (for a sufficiently small fluctuation). The fluctuation-dissipation theorem enables transport coefficients to be calculated in terms of response to external fields.

*Subjects:*
Chemistry — Physics.