  Exact values for bulk and shear viscosity are important to characterize a
fluid and they are a necessary input for a continuum description. Here we
present two novel methods to compute bulk viscosities by non-equilibrium
molecular dynamics (NEMD) simulations of steady-state systems with periodic
boundary conditions -- one based on frequent particle displacements and one
based on the application of external bulk forces with an inhomogeneous force
profile.
  In equilibrium simulations, viscosities can be determined from the stress
tensor fluctuations via Green-Kubo relations; however, the correct
incorporation of random and dissipative forces is not obvious. We discuss
different expressions proposed in the literature and test them at the example
of a dissipative particle dynamics (DPD) fluid.
