The analysis of transport properties stability in molecular dynamics simulations

Abstract

The Green-Kubo time correlation function is used to predict fluid argon transport properties, such as diffusion coefficient, viscosity and thermal conductivity, through molecular dynamics simulations. The results show that the transport characteristics, especially the viscosity and thermal conductivity, fluctuate intensely during the simulations. The collective stress tensor is separated into two parts, one is due to the kinetic energy and the other is due to the pair virial function, and the collective heat flux vector is contributed from the kinetic energy, the intermolecular potential and the pair virial function. The results show that the transport characteristics, especially the viscosity and the thermal conductivity, fluctuate intensely during the simulations. The most important contribution to the viscosity and the thermal conductivity is from the autocorrelation of the virial term. The calculations indicate that a more compatible integration time step method is needed to reduce instabilities when the Green-Kubo time correlation is used to calculate the fluid transport parameters. Other factors which influence the stability are also discussed in the paper.