The role of the dissipative and random forces in the calculation of the pressure of simple fluids with dissipative particle dynamics

TL;DR

This study investigates how viscous and Brownian forces influence the pressure in dissipative particle dynamics simulations, finding their contribution negligible under proper conditions, and explores system behavior when random forces are absent.

Contribution

It demonstrates that dissipative and random forces have minimal impact on pressure in long simulations when properly coupled, and analyzes system freezing without random forces.

Findings

Random and dissipative forces contribute negligibly to pressure in long simulations.

Proper coupling of forces ensures accurate pressure calculations.

System freezes when random forces are removed, due to force competition.

Abstract

The role of viscous forces coupled with Brownian forces in momentum conserving computer simulations is studied here in the context of their contribution to the total average pressure of a simple fluid as derived from the virial theorem, in comparison with the contribution of the conservative force to the total pressure. The specific mesoscopic model used is the one known as dissipative particle dynamics, although our conclusions apply to similar models that obey the fluctuation dissipation theorem for short range interactions and have velocity dependent viscous forces. We find that the average contribution of the random and dissipative forces to the pressure is negligible for long simulations, provided these forces are appropriately coupled and when the finite time step used in the integration of the equation of motion is not too small. Finally, we study the properties of the fluid when the random force is made equal to zero and find that the system freezes as a result of the competition of the dissipative and conservative forces.