Effect of softness of the potential on the stress anisotropy in liquids

We have performed molecular dynamics simulations of dense liquids using nonconformal and Gaussian potential models. We investigate the effect of the softness of the potential on the pressure tensor of liquids and dense fluids when the simulations are carried out using parallelepiped cells. The combination of periodic boundary conditions and small cross sectional areas induces an anisotropy in the diagonal components of the pressure tensor. This anisotropy results in an artificial stress in the system that has to be taken into account in simulations of explicit interfaces, where the artificial stress introduces errors in the computation of the surface tension. At high liquid densities the stress anisotropy exhibits an oscillatory dependence with the cross sectional area of the simulation box. We find that the softness of the potential has a dramatic effect on the amplitude of the oscillations, which can be significantly reduced in soft potentials, such as those used in the modeling of hydrocarbon liquids or polymers.