We present potential energy surfaces for Rh-CO obtained from density functional theory for two electronic states of Rh-CO. We have performed local spin-density calculations including relativistic as well as gradient corrections. The construction of a reasonably accurate atom-atom potential for Rh-CO is not possible. We were much more successful in constructing the potential energy surfaces by representing the potential as a spherical expansion. The expansion coefficients, which are functions of the distance between the rhodium atom and the carbon monoxide center of mass, can be represented by Lennard-Jones, Buckingham, or Morse functions, with an error of the fit within 10 kJ/mol. The potential energy surfaces$ using Morse functions, predict that the electronic ground state of Rh-CO is 2 ' or 'A. This is a linear structure with an equilibrium distance of rhodium to the carbon monoxide center of mass of 0.253 nm. The bonding enerq is -184 kJ/mol. Further, Morse functions predict that the first excited state is A'. This is a bent structure (LRh-CO = 14" ) with an equilibrium distance of rhodium to the carbon monoxide center of mass of 0.298 nm. The bonding energy of this state is -60 kJ/mol. Both these predictions are in good agreement with the agtual density functional calculations. We fymd 0.250 nm with -205 kJ/mol for Z+ and 0.253 nm with -199 kJ/mol for A. For 4A', we found 0.271 nm, LRh-CO = 30", with -63 kJ/mol. The larger deviation for :A' than for 'Z' or *A is a consequence of the fact that the minimum for A' is a very shallow well. 0 1994 by John Wiley & Sons, Inc. nological importance and of great catalytic interest. Examples of where this interaction plays a crucial role are the Fischer-Tropsch synthesis of hydrocarbons,' the catalytic production of methane: and automobile exhaust ~atalysis.~ Because of this, the interaction between carbon monoxide and transition metals is both experimen-