Recovering Pressure-Independent Dilute-Gas Thermal Conductivity from Molecular Dynamics: Green-Kubo Sampling...

Gas thermal conductivity is important in many engineering systems, but reliable data are not available for every gas, mixture, temperature, and pressure. Molecular dynamics can help fill this gap by predicting transport directly from molecular motion. However, gas simulations face time- and length-scale limitations that are different from those commonly encountered in liquids and solids. These limitations are not always recognized, and standard molecular dynamics tools are often used without checking whether their settings are suitable for dilute gases. This study uses the pressure-independent thermal conductivity of dilute argon as a benchmark for evaluating equilibrium and nonequilibrium molecular dynamics methods. At fixed temperature, changing pressure changes the mean free path and collision time even though the bulk thermal conductivity remains constant. Green-Kubo calculations at 300 K and 1 atm produced values from 0.009 to 0.022 W m^-1 K^-1 as the sampling interval and correlation length were varied, compared with the expected value of about 0.0175 W m^-1 K^-1. Some settings reproduced the expected value without showing convergence, demonstrating the risk of using Green-Kubo as a black-box calculation. NEMD simulations from 0.2 to 1.0 atm showed that the apparent conductivity increased with longitudinal system size and approached similar asymptotic values across pressure. The results recover pressure-independent gas thermal conductivity while showing that decreasing pressure increases the molecular length and time scales relevant to MD simulations. Reliable gas-property prediction therefore requires direct checks of Green-Kubo time scales and NEMD system-size dependence.