Extension of the Fluctuation-Dissipation Theorem to the Physical Aging of a Model Glass-Forming Liquid

We present evidence in favor of the possibility of treating an out-of-equilibrium supercooled simple liquid as a system in quasiequilibrium. Two different temperatures, one controlled by the external bath and one internally selected by the system, characterize the quasiequilibrium state. The value of the internal temperature is explicitly calculated within the inherent structure thermodynamic formalism. We find that the internal temperature enters the relation between the response to an external perturbation and the long-time decay of fluctuations in the liquid.