For a liquid above its melting point (Tm), the activation energy determined at constant density, E∞(ρ), is associated with “true” activated dynamics, while the ratio E∞(ρ)/E∞(P), where E∞(P) is the activation energy determined at constant pressure, is a measure of the relative importance of activation dynamics. If E∞(ρ)≫Tm, the liquid is called “strong,” and for strong liquids it appears that E∞(ρ)/E∞(P)>0.5; these inequalities taken together indicate relaxations dominated by activated dynamics. If E∞(ρ)≈Tm, the liquid is called “weak,” and for weak liquids E∞(ρ)/E∞(P)<0.5; these inequalities taken together indicate relaxations dominated by nonactivated processes. Although E∞(ρ) can be big or small, E∞(P) seems to be appreciably larger than Tm. Many glass-formers are quite “strong,” even some such as orthoterphenyl, which is normally classified as “fragile,” and, consequently, the relaxations of these are likely to be dominated by activated dynamics over the entire temperature range from well above melting down to the glass transition; the relaxation of others, such as toluene, seem not to be activated at temperatures above melting.

Ferrer, M. L., & Kivelson, D. (1999). Constant density activation energies and the role of activated dynamics in liquids above their melting points. The Journal of Chemical Physics, 110(22), 10963–10968.