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

In this work, we focused on the theoretical study of the structural properties of non-additive hard-body models with applications to polar and associating liquids. The study builds upon earlier simulation results for simple fluids, where hard-body models have been commonly used as reference systems for constructing molecular equations of state. While this approximation has proven successful for simple liquids, its direct application to polar or associating fluids has not been common practice. The standard approach typically approximates molecules as fused hard spheres, whose effective diameters are derived from realistic interaction models. The interaction between such hard spheres is then treated as additive, with the effective contact distance defined as the arithmetic mean of the radii of the interacting particles.

In this work, we demonstrate that assigning a unique effective hard-sphere diameter to each pair of interacting particles is possible based on the original (realistic) model, without relying on additive rules. This approach captures more details of the underlying interaction. We show that such non-additive models provide a better representation of the structure of the studied fluids and can also capture, to some extent, their orientational correlations. The results suggest that non-additive hard-body models may serve as suitable reference systems even for real polar and associating liquids within the framework of perturbation theories.

Illustration of the interaction of two acetonitrile molecules. (Left) realistic model, (right) derived model of fused hard spheres. Additive interactions are shown by solid lines, dashed lines indicate partial non-additive diameters of hard spheres

• Škvára J.*, Nezbeda I.: Structure of liquids from reference hard body fluids: Additive vs non-additive hard body models. J. Chem. Phys. 2025, 162(16), 164505. doi.org/10.1063/5.0253528