Purely viscous fluids

It is shown that fluids consisting of particles that interact with potentia ls containing an accessible hard core have elastic moduli at infinite frequ ency that are infinite in magnitude. A consequence of this is that such a f luid, at any density or temperature, is not viscoelastic as are all real fl uids. The viscosity of the fluid, whether shear or bulk, remains finite at all shear rates and the fluid never becomes elastic even for applied stress es at infinitely high frequency. Some care is required, however, in definin g what is meant by 'hard core' and 'accessible'. The conclusions are suppor ted by the results of molecular dynamics calculations of the viscoelastic b ehaviour of fluids interacting with steeply repulsive pair potentials.