Computation of the viscosity of a liquid from time averages of stress fluctuations - art. no. 011207

The shear viscosity can be calculated from the standard deviation of an equ ilibrium ensemble of time averages of the shear stress computed along finit e duration phase space trajectory segments. The mean square of the segment averages of the sheer stress is proportional to the shear viscosity and inv ersely proportional to the duration of the trajectory segments and the numb er of particles. We test the fluctuation relation for the shear viscosity a nd show that it provides a simple but viable means of computing the zero st rain rate shear viscosity. We decompose the shear viscosity computed using this fluctuation method, into its "kinetic" and "configurational" component s. We also calculate the relevant relaxation times. We compare the computed results with standard nonequilibrium molecular dynamics simulations. Final ly we compute the bulk viscosity using an analogous fluctuation method.