This paper presents an ab initio database of CO chemisorption energies on various metal surfaces and overlayers, including Ni(111), Cu(111), Ru(0001), Pd(111), Ag(111), Pt(111), Au(111), and Cu3Pt(111). The results are explained using a simple model that describes the interaction between the metal d states and the CO 2π* and 5σ states, renormalized by the metal sp continuum. The model rationalizes the results of Rodriguez and Goodman, showing a strong correlation between CO chemisorption energy and surface core level shift. The paper discusses the physics of CO adsorption on metal surfaces and overlayers, using an extensive ab initio database of CO chemisorption energies calculated within density functional theory (DFT) using the generalized gradient approximation (GGA). It demonstrates that the trends in the database can be understood using a simple two-level model describing the coupling of the CO 5σ and 2π* states to the metal d valence states. A key surface parameter determining the strength of the bonding is the energy of the center of the metal d band. This surface property can be obtained from spectroscopic methods. The paper also discusses the electronic structure of CO adsorption on metal surfaces, showing that the filled 5σ and doubly degenerate, empty 2π* electronic states are mainly responsible for bonding to metal surfaces. The model expression for the d contribution to the CO chemisorption energy over transition metal surfaces is derived. The model is shown to account for experimental data of Rodriguez and Goodman. The paper concludes that the interaction between the metal d states and the CO 2π* and 5σ states is responsible for the trends in CO chemisorption energies over the wide range of late transition metal surfaces considered, as well as for the details for metallic overlayers and alloy surfaces. The model is in complete agreement with the theoretical interpretations developed by Blyholder, Bagus, and others. The paper also discusses the correlation between surface core level shifts and CO chemisorption energy, showing that the variation in the surface core level shifts can be viewed as a measure of the variation in the d band center. The paper concludes that the model can account for the main trends in CO bonding from one surface to the next.This paper presents an ab initio database of CO chemisorption energies on various metal surfaces and overlayers, including Ni(111), Cu(111), Ru(0001), Pd(111), Ag(111), Pt(111), Au(111), and Cu3Pt(111). The results are explained using a simple model that describes the interaction between the metal d states and the CO 2π* and 5σ states, renormalized by the metal sp continuum. The model rationalizes the results of Rodriguez and Goodman, showing a strong correlation between CO chemisorption energy and surface core level shift. The paper discusses the physics of CO adsorption on metal surfaces and overlayers, using an extensive ab initio database of CO chemisorption energies calculated within density functional theory (DFT) using the generalized gradient approximation (GGA). It demonstrates that the trends in the database can be understood using a simple two-level model describing the coupling of the CO 5σ and 2π* states to the metal d valence states. A key surface parameter determining the strength of the bonding is the energy of the center of the metal d band. This surface property can be obtained from spectroscopic methods. The paper also discusses the electronic structure of CO adsorption on metal surfaces, showing that the filled 5σ and doubly degenerate, empty 2π* electronic states are mainly responsible for bonding to metal surfaces. The model expression for the d contribution to the CO chemisorption energy over transition metal surfaces is derived. The model is shown to account for experimental data of Rodriguez and Goodman. The paper concludes that the interaction between the metal d states and the CO 2π* and 5σ states is responsible for the trends in CO chemisorption energies over the wide range of late transition metal surfaces considered, as well as for the details for metallic overlayers and alloy surfaces. The model is in complete agreement with the theoretical interpretations developed by Blyholder, Bagus, and others. The paper also discusses the correlation between surface core level shifts and CO chemisorption energy, showing that the variation in the surface core level shifts can be viewed as a measure of the variation in the d band center. The paper concludes that the model can account for the main trends in CO bonding from one surface to the next.