Thermostatting nonequilibrium systems: approximating the limits of quasiequilibrium through simple molecular dynamics simulations

To replicate experimental conditions, Molecular Dynamics (MD) simulations tend to employ thermostats – such as the Nosé-Hoover, Nosé-Hoover Chains, and Langevin thermostats – to maintain the temperature of systems. Thermostats have also often been used in non-equilibrium MD (NEMD) as dissipative tools to achieve steady-states. While their appropriateness in equilibrium – assessed by their ability to reproduce the canonical ensemble – has been demonstrated, their aptness for nonequilibrium has not been proven. In this work it is argued that, due to the fluctuation-dissipation theorem, only the quasiequilibrium regime inherits the justification for using these thermostats, making an approximation of the limits of quasiequilibrium imperative. Through the study of a simple model, an approximate bound for the quasiequilibrium regime for perturbed thermostatted systems is revealed. Along the way, a categorization of models for external perturbations is given.