Calculated CO stretching frequencies for the Rh(111)-(2 x 2)-3CO structure are compared with recent high-resolution electron energy loss spectroscopy (HREELS) experiments. Two loss peaks in the HREELS spectrum at 2070 and 1861 cm(-1) were attributed to ontop- and hollow-bonded CO molecules, respectively. Two weaker peaks, which had not been observed before due to the lack of resolution, appeared at 1785 and 1925 cm(-1). DFT calculations reveal that the loss features at 1861 and 1785 cm(-1) originate from the adsorption of CO molecules at hcp and fcc sites and correspond to the in-phase and out-of-phase normal modes resulting from dipole-dipole coupling. (C) 2002 Elsevier Science B.V. All rights reserved.