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For a stationary zero-mean velocity with an absolutely integrable velocity autocorrelation function $\chi$, the diffusion coefficient is its time integral. Indeed, differentiation of the mean squared displacement identity gives $\frac{1}{2}\frac{d}{dt}\mathbb{E}[(X(t)-X(0){)}^2]={\int }_0^t\chi (u)du$, and the right side tends to the displayed integral. In the equilibrium generalized Langevin equation, this gives $D=\Theta /\tilde{\kappa }(0)$ whenever stable relaxation and the required integrability hold. The formula is named after Melville S. Green and Ryogo Kubo.