  We show that the long time tail of the shear stress autocorrelation, whose
growth at large supercooling is responsible for the apparent divergence of the
shear viscosity, is a direct result of a residual shear stress in the
structures associated with the local potential minima. We argue that the
essential mechanical transition experienced by a liquid on cooling occurs at a
temperature well above the glass transition temperature and corresponds to the
crossover from the high temperature liquid to the viscous liquid, the latter
characterised by stress relaxation dominated by the residual stress. Following
on from this observation, as the density is decreased, the local potential
minima become unable to sustain any persistent stress (and, hence, support a
glass transition), in a manner that can be explicitly connected to the
interactions between atoms. The reported crossover implies an associated change
in the mechanism of dissipation in liquids and, hence, raises the prospect of a
coherent microscopic treatment of nonlinear rheology and the relationship
between self diffusion and viscosity in supercooled liquids.
