Fluids of hard diatomic molecules. II

We consider fluids of hard diatomic molecules composed of two fused hard spheres of unequal diameter. The orientation average of the Mayer f function and the second virial coefficient B2 are calculated exactly by analytic integration in closed form. The calculation uses a decomposition of f(12) into five terms. The only difficult term is integrated in tetrahedral coordinates. Agreement with previous numerical results for B2 is excellent. Two of the five terms make small to moderate contributions to B2, depending on the molecular parameters. This supports the view that the decomposition is a useful device in formulating approximate theories. The integrals evaluated for the diatomic case are used to obtain B2 and the orientation-averaged f(12) exactly for a class of linear symmetric triatomic molecules.