The effect of promoters or inhibitors on the vibrational frequency of CO chemisorbed on transition-metal surfaces has been investigated by means of cluster models in which the coadsorbed electropositive or electronegative atoms have been replaced by point charges. Ab initio Hartree-Fock wave functions have been determined for CO chemisorbed on small Cu and Pd clusters in the presence of point charges as well as of uniform electric fields of different magnitude. It is shown that the simple electrostatic interaction between the point charges and the CO dipole makes an important contribution to the large vibrational shifts observed experimentally. Further contributions to the shifts occur because the cluster electron distribution polarizes in response to the field created by the point charges. These electronic effects, however, are much less important than the electrostatic contribution. The dependence of both the electrostatic, or Stark, effects and the electronic, or chemical, effects on field strength is studied by placing the cluster in uniform electric fields of different magnitude. As the field strength is increased, the electronic effect on the frequency shift becomes larger but, even for the largest fields considered here, it is always smaller than the Stark shift. The frequency shifts due to the point charges are shown to correspond to those which would be caused by a large uniform field. © 1989 The American Physical Society.