From equilibrium to nonequilibrium statistical mechanics of liquids

Relevant and fundamental concepts of the statistical mechanical theory of classical liquids are ordinarily introduced in the context of the description of thermodynamic equilibrium states. This makes explicit reference to probability distribution functions of equilibrium statistical ensembles (canonical, microcanonical, etc.) in the derivation of general and fundamental relations between interparticle interactions and measurable macroscopic properties of a given system. This includes, for instance, expressing the internal energy and the pressure as functionals of the radial distribution function, or writing transport coefficients (diffusion constant, linear viscosity, etc.) in terms of integral relations involving both static and dynamic autocorrelation functions (density-density, stress-stress, etc.). Most commonly, however, matter is not in thermodynamic equilibrium, and this calls for the extension of these relations to out-of-equilibrium conditions with the aim of understanding, for example, the time-dependent transient states during the process of equilibration, or the aging of glass- and gel-forming liquids during the formation of nonequilibrium amorphous solid states. In this work, we address this issue from both a general perspective and an illustrative concrete application focused on the first-principles description of rheological and viscoelastic properties of glass- and gel-forming liquids.