The fluctuation-dissipation theorem relates the correlation function of the observable of interest ⟨ ⟨ x ^ ^ ( t ) x ^ ^ ( 0 ) ⟩ ⟩ (a measure of fluctuation) to the imaginary part of the response function Im [ χ χ ( ω ω ) ] = [ χ χ ( ω ω ) − − χ χ ∗ ∗ ( ω ω ) ] / 2 i in the frequency domain (a measure of dissipation). A link between these quantities can be found through the so-called Kubo formula