Equilibrium and out of equilibrium thermodynamics in supercooled liquids   and glasses
  We review the inherent structure thermodynamical formalism and the formulation of an equation of state for liquids in equilibrium based on the (volume) derivatives of the statistical properties of the potential energy surface. We also show that, under the hypothesis that during aging the system explores states associated to equilibrium configurations, it is possible to generalize the proposed equation of state to out-of-equilibrium conditions. The proposed formulation is based on the introduction of one additional parameter which, in the chosen thermodynamic formalism, can be chosen as the local minima where the slowly relaxing out-of-equilibrium liquid is trapped. 
S. Mossa, E. La Nave, P. Tartaglia, and F. Sciortino