The chemisorptive interaction of the CO/Cu(100) system is studied with a Cu5(1,4)CO cluster model using the constrained space orbital variation (CSOV) analysis. This analysis has been used previously to examine the origin of the CO-metal bond and to show that the energetic importance of the metal to CO π donation is larger than that of the CO to metal σ donation. In the present work, we use it to determine the origin of the differential CO 5σ ionization potential shift observed in the photoemission spectra of CO/Cu(100). This shift is shown to be largely due to electrostatic, environmental potential effects. It does not indicate the nature of the CO-metal bond but does indicate the CO adsorption geometry. © 1985.