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This paper derives and discusses the configuration-space Langevin equationdescribing a physically aging R-simple system and the correspondingSmoluchowski equation. Externally controlled thermodynamic variables liketemperature, density, pressure enter the description via the single parameter${T}_{\rm s}/T$ in which $T$ is the bath temperature and ${T}_{\rm s}$ is the"systemic" temperature defined at any time $t$ as the thermodynamic equilibriumtemperature of the state point with density $\rho(t)$ and potential energy$U(t)$. In equilibrium ${T}_{\rm s}\cong T$ with fluctuations that vanish inthe thermodynamic limit. In contrast to Tool's fictive temperature and othereffective temperatures in glass science, the systemic temperature is definedfor any configuration with a well-defined density, even if it is not in anysense close to equilibrium. Density and systemic temperature define an agingphase diagram in which the aging system traces out a curve. Predictions arediscussed for aging following various density-temperature andpressure-temperature jumps from one equilibrium state to another, as well asfor a few other scenarios. The proposed theory implies that R-simpleglass-forming liquids are characterized by a dynamic Prigogine-Defay ratio ofunity.

Isomorph theory of physical aging