  We have used first-principles calculations to study the structural,
electronic, and thermodynamic properties of the three known forms of Rh-S
chalcogenides: Rh$_2$S$_3$, Rh$_3$S$_4$, and Rh$_{17}$S$_{15}$. Only the first
of these materials of interest for catalysis had been studied previously within
this approach. We find that Rh$_{17}$S$_{15}$ crystallizes in a {\em
Pm}\={3}{\em m} centrosymmetric structure, as believed experimentally but never
confirmed. We show band structures and densities of states for these compounds.
Finally, we also investigated the ground state structure of solid sulfur
($\alpha$-S), one of the few elements that represents a challenge for full
first-principles calculations due to its demanding 128-atom unit cell and
dispersion interactionis between S$_8$ units.
