  We describe a method to determine whether a material has isomorphs in its
thermodynamic phase diagram. Isomorphs are state points for which various
properties are invariant in reduced units. Such materials are commonly
identified from strong correlation between thermal fluctuations of the
potential energy, U, and the virial W, but this identification is not generally
applicable to real materials. We show from molecular dynamic simulations of
atomic, molecular, and polymeric materials that systems with strong U-W
correlation cannot be pressure densified; that is, the density obtained on
cooling to the glassy state and releasing the pressure is independent of the
pressure applied during cooling.
