On the generalized bulk viscosity behavior

We have calculated generalized longitudinal, eta (L)(k), shear, eta (S)(k), and bulk, eta (B)(k) viscosity of argon at two thermodynamic states by mol ecular-dynamics simulation. Bulk viscosity has been evaluated with two inde pendent methods that give results in good agreement. We find that bulk visc osity stays positive at all wave numbers k, and show that negative values o f eta (B)(k) are due to a straightforward (and incorrect) extension to fini te k's of the relation eta (B)(k) = eta (L)(k)-4 eta (S)(k)/3, only valid a t vanishing k's. (C) 2001 American Institute of Physics.