Green–Kubo relations
The Green–Kubo relations (Melville S. Green 1954, Ryogo Kubo 1957) give the exact mathematical expression for a transport coefficient γ {\displaystyle \gamma } in terms of the integral of the equilibrium time correlation function of the time derivative of a corresponding microscopic variable A {\displaystyle A} (sometimes termed a "gross variable", as in ): γ = ∫ 0 ∞ ⟨ A ˙ ( t ) A ˙ ( 0 ) ⟩ d t . {\displaystyle \gamma =\int _{0}^{\infty }\left\langle {\dot {A}}(t){\dot {A}}(0)\right\rangle \;{\mathrm {d} }t.} One intuitive way to understand this relation is that relaxations resulting from random fluctuations in equilibrium are indistinguishable from those due to an external perturbation in linear response.