Non-equilibrium generalized Langevin equation for multi-dimensional observables

TL;DR

This paper derives a non-equilibrium generalized Langevin equation for multi-dimensional observables using the Mori-Zwanzig formalism, capturing both Markovian and non-Markovian forces, with applications to biological systems like protein folding.

Contribution

It introduces a non-equilibrium Mori GLE for multi-dimensional observables with a time-dependent Hamiltonian, expanding the theoretical framework for complex system modeling.

Findings

Derived a non-Markovian GLE for multi-dimensional observables.

Identified conditions where the Markovian force simplifies to an instantaneous friction.

Applied the framework to biological protein folding processes.

Abstract

The Mori-Zwanzig formalism is a powerful theoretical framework for deriving equations of motion for coarse-grained observables in the form of generalized Langevin equations (GLEs) involving evolution and projection operators. Using a time-dependent many-body Hamiltonian and a multi-dimensional Mori projection operator, we derive a non-equilibrium Mori GLE for a multi-dimensional observable of interest $\vec{A}$ that consists of a Markovian force, a running integral over time of a non-Markovian friction force, and an orthogonal force that is often interpreted as a random force. We study the structure of the derived GLE in three limiting cases: when the components of $\vec{A}$ are uncorrelated, when the Hamiltonian is time-independent and thus the system is at equilibrium, and when both conditions are simultaneously satisfied. We highlight the presence of a contribution to the Markovian force that takes the form of an instantaneous friction force which only vanishes when the components of $\vec{A}$ are uncorrelated. Our non-Markovian framework is an important step towards the systematic modeling of the coupled kinetics of coarse-grained reaction coordinates in biological complex systems, exemplified for the coupled intra- and inter-protein folding during fibril formation of the human islet amyloid polypeptide (IAPP).