Fluctuation-dissipation theorem
= Fluctuation-dissipation theorem
{c}
{wiki}
The fluctuation-dissipation theorem relates an equilibrium correlation function to the dissipative part of the corresponding linear response. If $\rho=-2\operatorname{Im}G_R$, one common convention is $S(\omega)=\rho(\omega)/(1-e^{-\omega/T})$, which becomes $S(\omega)\simeq T\rho(\omega)/\omega$ in the classical limit $|\omega|\ll T$.