Glass-forming liquids: one or more 'order' parameters?

We first summarize the classical arguments that the vast majority of glass-forming liquids require more than one 'order' parameter for their description. Critiques against this conventional wisdom are then presented, and it is argued that the matter deserves to be reconsidered in the light of recent experimental developments. Out of the eight basic thermoviscoelastic frequency-dependent response functions, there are generally three independent functions. For stochastic dynamics we show that there are only two independent response functions; for this case it is shown how analytic continuation may be utilized to express the third response functions in terms of two others. Operational criteria are presented for the linear thermoviscoelasticity being described by a single 'order' parameter, in which case there is just one independent thermoviscoelastic response function. It is shown that a description with a single 'order' parameter applies to a good approximation whenever thermal equilibrium fluctuations of fundamental variables like energy and pressure are strongly correlated. Results from computer simulations showing that this is the case for a number of simple glass-forming liquids, as well as a few exceptions, are briefly presented. Finally, we briefly discuss a new conjecture according to which experiments at varying temperature and pressure follow the density scaling expression for the relaxation time, tau = F(rho(x)/ T) (rho and T are density and temperature), if and only if the liquid is 'strongly correlating', i.e., to a good approximation is described by a single 'order' parameter.