Projection-operator formalism and coarse-graining

TL;DR

This paper derives a generalized Langevin equation using a projection operator formalism, emphasizing its advantages for non-equilibrium systems and providing a methodology for systematic coarse-graining without thermodynamic-limit restrictions.

Contribution

It introduces a refined projection-operator approach for non-equilibrium thermodynamics, enabling systematic coarse-graining and microcanonical entropy definition based on chosen variables.

Findings

Formalism better suited for non-equilibrium systems

Provides a coarse-graining methodology applicable beyond thermodynamic limits

Equivalence with homogenization theory in linear limit

Abstract

A careful derivation of the generalized Langevin equation using "Zwanzig flavor" projection operator formalism is presented. We provide arguments why this formalism has better properties compared to alternative projection-operator formalisms for deriving non-equilibrium, non-thermodynamic-limit, equations. The two main ingredients in the derivation are Liouville's theorem and optimal prediction theory. As a result we find that equations for non-equilibrium thermodynamics are dictated by the formalism once the choice of coarse-grained variables is made. This includes a microcanonical entropy definition dependent on the coarse-grained variables. Based on this framework we provide a methodology for succesive coarse-graining. As two special cases, the case of linear coefficients and coarse-graining in the thermodynamic limit are treated in detail. In the linear limit the formulas found are equivalent with those of homogenization theory. In this framework there are no restrictions with respect to the thermodynamic-limit or nearness to equilibrium. We believe the presented approach is very suitable for the development of computational methods by means of coarse-graining from a more detailed level of description.