On the equation of state of hard chain molecules

The chain-of-rotators (COR) equation of state developed by Chien, Greenkorn, and Chao makes an assumption on the hard chain partition function based on results for monomers and dimers, i.e., spheres and dumbbells. Here this assumption is checked for trimers by comparison with the results of Boubik's hard convex body equation. It turns out that the COR assumption is, in general, a good approximation. Its quality depends somewhat on the bond length and the bond angle which it does not consider. The agreement is especially good for angles somewhat less than 90°.