Delaunay Simplexes in Liquid Cyclohexane

We study the structure of computer models of liquid cyclohexane and its noncyclic analogue 2,3-dimethylbutane. It is shown that regardless of the different chemical structure of these molecules, they both behave in liquid like spherical particles. It emerges in similarity of pair correlation functions calculated for centers of mass of molecules with the functions for simple liquids. An analogy with simple liquids also follows from the analysis of Delaunay simplexes. Structural difference between our molecular liquids is the same as between disordered packings of spheres with corresponding densities. More dense systems contain more Delaunay simplexes of shape close to the perfect tetrahedron. They associate by faces and produce clusters with morphology alien for crystalline lattices, Phys. Rev. Lett 98, 235504 (2007). Thus the geometrical principle for the formation of non-crystalline packings is valid not only for spherical atoms or hard spheres, where the tetrahedron is a natural geometrical element, but also for molecules of more complex shape, which, because of thermal motion and lack of significant orientational correlation occupy a spherical volume only on average.