The key feature of glass forming liquids is the super-Arrhenian temperature dependence of the mobility, where the mobility can increase by ten orders of magnitude or more as the temperature is decreased if crystallization does not intervene. A fundamental description of the super-Arrhenian behavior has been developed; specifically, the logarithm of the relaxation time is a linear function of $1/{\overline{U}}_x$, where ${\overline{U}}_x$ is the independently determined excess molar internal energy and $B$ is a material constant. This one-parameter mobility model quantitatively describes data for 21 glass forming materials, which are all the materials where there are sufficient experimental data for analysis. The effect of pressure on the $loga$ mobility is also described using the same ${\overline{U}}_x(T,p)$ function determined from the difference between the liquid and crystalline internal energies. It is also shown that $B$ is well correlated with the heat of fusion. The prediction of the $B/{\overline{U}}_x$ model is compared to the Adam and Gibbs $1/T{\overline{S}}_x$ model, where the $B/{\overline{U}}_x$ model is significantly better in unifying the full complement of mobility data. The implications of the $B/{\overline{U}}_x$ model for the development of a fundamental description of glass are discussed.

• Received 21 February 2018
• Revised 30 April 2018

DOI:https://doi.org/10.1103/PhysRevMaterials.2.055604