The role of the excluded volume in the molecular dynamics for molecular systems revealed by the direct computational approach

• Excluded volume estimated using minimal distances as a state-dependent parameter. • Values of the excluded volumes significantly smaller than those determined by the typical theoretical models. • Excluded to free volume ratio as a constant at the isomorphic conditions. In this paper, we directly verify the methods of determining the free volumes proposed in the literature. Using the computer simulations of the molecular dynamics for two quasi-real model systems we estimate, in the possibly most direct way, the excluded volume, which is a quantity crucial for the calculation of the free volume. The values of the excluded volume obtained by us turn out to be much lower than those calculated by routinely used and recently proposed methods in the literature. Moreover, our results suggest that, in contrast to the typical theoretical models, the excluded volume is a state-dependent parameter. We also show that the dynamic properties of the systems plotted versus the ratio of the excluded to the free volumes determined by our approach collapsed onto one curve, which is in line with the isomorphs theory.