 Generalized fluctuation-dissipation relation

For the equilibrium generalized Langevin equation with unit mass, the random force has zero mean and the displayed autocorrelation, where $\Theta$ is the equilibrium velocity variance. If the deterministic relaxation $g$ is stable, $g(0)=1$ and $g^{\prime }=-\kappa \ast g$, then the stationary velocity autocorrelation function is $\chi =\Theta g$, equivalentlyOne proof uses the power spectral density. The even force autocorrelation has spectrum $2\Theta \mathrm{Re}\tilde{\kappa }(i\omega )$. With transfer function $H(i\omega )=[i\omega +\tilde{\kappa }(i\omega ){]}^{-1}$, the velocity spectrum is $2\Theta |H{|}^2\mathrm{Re}\tilde{\kappa }=2\Theta \mathrm{Re}H$, the spectrum of the even extension of $\Theta g$. Stability removes initial transients with finite second moment. A valid force autocorrelation must have nonnegative power spectral density; an arbitrary proposed memory kernel does not automatically satisfy this condition.