Structure of liquids from reference hard body fluids: Additive vs non-additive hard body models

Following the early simulation results for simple liquids, various hard body fluids have been used in molecular-based equations of state as a leading/reference term. This has been justified for normal liquids by the similarity of their structure, but for polar and associating ones, a direct application of hard body models has not been considered so far. Viewing the hard body models, fused-hard-sphere bodies, as simple geometrical objects, their mutual interaction is additive. However, when accounting for the mutual effect of the site-site interactions, the individual hard sphere-hard sphere interactions may become non-additive, and consequently, the resulting interaction between the hard bodies becomes non-additive, which may also affect their structure. The effect of the non-additivity on the structural properties, the site-site and dipole-dipole correlation functions are analyzed in detail by considering three polar fluids, quadrupolar carbon dioxide, dipolar acetonitrile, and acetone, as well as two associating fluids, methanol and water. The modification of the mutual geometry in the non-additive models leads to differences both in their structural and orientation correlations. The comparison of the structure of the non-additive purely repulsive hard-body models with those of the empirical models of the chosen real liquids shows surprising similarities, which extends the possibilities of the direct application of the hard-body fluids as reference systems in perturbation theories.