Nature of intrinsic uncertainties in equilibrium molecular dynamics estimation of shear viscosity for simple and complex fluids
Abstract
We study two types of intrinsic uncertainties, statistical errors and system size effects, in estimating shear viscosity via equilibrium molecular dynamics simulations, and compare them with the corresponding uncertainties in evaluating the selfdiffusion coefficient. Uncertainty quantification formulas for the statistical errors in the shearstress autocorrelation function and shear viscosity are obtained under the assumption that shear stress follows a Gaussian process. Analyses of simulation results for simple and complex fluids reveal that the Gaussianity is more pronounced in the shearstress process (related to shear viscosity estimation) compared with the velocity process of an individual molecule (related to selfdiffusion coefficient). At relatively high densities corresponding to a liquid state, we observe that the shear viscosity exhibits complex sizedependent behavior unless the system is larger than a certain length scale, and beyond which, reliable shear viscosity values are obtained without any noticeable scaling behavior with respect to the system size. We verify that this sizedependent behavior is configurational and relate the characteristic length scale to the shearstress correlation length.
 Publication:

Journal of Chemical Physics
 Pub Date:
 July 2018
 DOI:
 10.1063/1.5035119
 arXiv:
 arXiv:1807.08063
 Bibcode:
 2018JChPh.149d4510K
 Keywords:

 Physics  Chemical Physics;
 Physics  Computational Physics;
 Physics  Fluid Dynamics
 EPrint:
 J. Chem. Phys. 149, 044510 (2018)