We have constructed a potential energy function to describe the interaction of CO molecules with each other and with the (100) surface of NaCl. It uses distributed multipoles to describe the electrostatic interactions, distributed polarizabilities to describe induction, anisotropic dispersion coefficients taken from work by Rijks and Wormer, and anisotropic atom-atom repulsion terms derived from ab initio intermolecular perturbation theory. We have also investigated the structure and charge distribution of the NaCl surface. We find that an isolated CO molecule on the NaCl(100) surface adopts a position with the CO axis perpendicular to the surface and the C atom over a surface Na+ ion, but in monolayers of CO the molecules are tilted by about 28 degrees. There are two 1X1 and two 2X1 structures, all with very similar energies; a further 1X1 structure with the molecules perpendicular to the surface is an index-2 stationary point with energy only 32 cm(-1) above the lowest-energy structure. The results are consistent with the experimental infrared spectra and with the measured enthalpy of adsorption.