A new kind of particle (body), which may form a reference fluid in the
thermodynamic perturbation theory of associating fluids, is introduce
d. The bodies are made up of real hard body cores with additional embe
dded interaction sites which only under certain circumstances interact
as hard spheres but are 'transparent' otherwise. The properties of th
e fluid of such bodies are exemplified by presenting the results for t
he structural and thermodynamic properties of the model of pseudo-hard
penta-atomics, i.e. hard spheres with four additional tetrahedrally a
rranged interaction sites.