Equilibration in the Nos\'e-Hoover isokinetic ensemble: Effect of inter-particle interactions

TL;DR

This paper studies how inter-particle interactions affect the equilibration process in Nosé-Hoover dynamics, revealing that long-range interactions cause extremely slow relaxation of momentum distributions, impacting the equivalence with canonical ensemble.

Contribution

It demonstrates the influence of interaction range on relaxation timescales in Nosé-Hoover dynamics, highlighting the role of long-range interactions in slowing convergence to equilibrium.

Findings

Equilibrium properties match canonical ensemble predictions.

Kinetic energy relaxes to target value over size-independent timescale.

Momentum distribution relaxation time diverges with system size for long-range interactions.

Abstract

We investigate the stationary and dynamic properties of the celebrated Nos\'e-Hoover dynamics of many-body interacting Hamiltonian systems, with an emphasis on the effect of inter-particle interactions. To this end, we consider a model system with both short- and long-range interactions. The Nos\'e-Hoover dynamics aims to generate the canonical equilibrium distribution of a system at a desired temperature by employing a set of time-reversible, deterministic equations of motion. A signature of canonical equilibrium is a single-particle momentum distribution that is Gaussian. We find that the equilibrium properties of the system within the Nos\'e-Hoover dynamics coincides with that within the canonical ensemble. Moreover, starting from out-of-equilibrium initial conditions, the average kinetic energy of the system relaxes to its target value over a {\it size-independent} timescale. However, quite surprisingly, our results indicate that under the same conditions and with only long-range interactions present in the system, the momentum distribution relaxes to its Gaussian form in equilibrium over a scale that {\it diverges with the system size}. On adding short-range interactions, the relaxation is found to occur over a timescale that has a much weaker dependence on system size. This system-size dependence of the timescale vanishes when only short-range interactions are present in the system. An implication of such an ultra-slow relaxation when only long-range interactions are present in the system is that macroscopic observables other than the average kinetic energy when estimated in the Nos\'e-Hoover dynamics may take an unusually long time to relax to its canonical equilibrium value. Our work underlines the crucial role that interactions play in deciding the equivalence between Nos\'e-Hoover and canonical equilibrium.