  In systems driven away from equilibrium, the velocity correlation function
and the linear response function to a small perturbation force do not satisfy
the fluctuation-response relation (FRR) due to the lack of detailed balance in
contrast to equilibrium systems. In this Letter, an equality between an extent
of the FRR violation and the rate of energy dissipation is proved for Langevin
systems under non-equilibrium conditions. This equality enables us to calculate
the rate of energy dissipation by quantifying the extent of the FRR violation,
which can be measured experimentally.
