Temperature of nonequilibrium steady-state systems

Area of Science:

• Thermodynamics
• Computational Physics
• Statistical Mechanics

Background:

• Nonequilibrium molecular dynamics (NEMD) simulations are crucial for studying systems under stress.
• NEMD models often employ synthetic thermostats to manage heat dissipation.
• Previous studies have highlighted discrepancies in temperature measurements within NEMD systems.

Purpose of the Study:

• To determine the operational temperatures of NEMD systems using a recently developed thermometer.
• To compare these operational temperatures with the dynamic temperatures of the same systems.
• To investigate the cause of temperature splitting and directional dependence in NEMD simulations.

Main Methods:

• Utilized a novel thermometer to measure operational temperatures in NEMD systems.
• Calculated dynamic temperatures, including kinetic and configurational components.
• Analyzed systems subjected to planar Couette flow and color flow.

Main Results:

• Operational temperatures were found to be closer to configurational temperatures than kinetic temperatures in dense liquids.
• A split in dynamic temperature (kinetic vs. configurational) and directional dependence were observed.
• These temperature variations were identified as artifacts of the instantaneous feedback in NEMD thermostats.

Conclusions:

• The observed temperature split and directional dependence in NEMD are artifacts of the simulation method.
• Realistic thermostats in real systems, even far from equilibrium, should yield practically identical and isotropic operational and dynamic temperatures.
• The findings suggest a need for re-evaluation of temperature definitions and measurements in NEMD simulations.