Perspective: Excess-entropy scaling

This article gives an overview of excess-entropy scaling, the 1977 discovery by Rosenfeld that entropy
determines properties of liquids like viscosity, diffusion constant, and heat conductivity. We give
examples from computer simulations confirming this intriguing connection between dynamics and
thermodynamics, counterexamples, and experimental validations. Recent uses in application-related
contexts are reviewed, and theories proposed for the origin of excess-entropy scaling are briefly
summarized. It is shown that if two thermodynamic state points of a liquid have the same microscopic
dynamics, they must have the same excess entropy. In this case, the potential-energy function exhibits
a symmetry termed hidden scale invariance, stating that the ordering of the potential energies of
configurations is maintained if these are scaled uniformly to a different density. This property leads to
the isomorph theory, which provides a general framework for excess-entropy scaling and illuminates,
in particular, why this does not apply rigorously and universally. It remains an open question whether
all aspects of excess-entropy scaling and related regularities reflect hidden scale invariance in one
form or other.