Intermolecular distance and density scaling of dynamics in molecular liquids

A broad variety of liquids conform to density scaling: relaxation times expressed as a function of the ratio of temperature to density, the latter raised to a material constant {\gamma}. For atomic liquids interacting only through simple pair potentials, the exponent {\gamma} is very nearly equal to n/3, where n is the steepness of the intermolecular potential, while for molecular liquids having rigid bonds and built using the same interatomic potential, {\gamma}>n/3. We find that for this class of molecular liquids {\gamma}=n/{\delta}, where the parameter {\delta} relates the intermolecular distance to the density along an isomorph (line of approximately constant dynamics and structure). {\delta} depends only on the molecular structure and not the interatomic potential.