Abstract Cluster model calculations have been performed to study coadsorption effects of CO and alkali atoms on Cu(100). Electronic, geometric and energetic effects have been investigated using methods including electron correlation. The main previously known properties of the coadsorption system are well reproduced by the calculations, such as a large lowering of the CO frequency and a lowering of the barrier for CO dissociation. An interesting effect found is that the alkali atoms are much less positive when they are adsorbed alone than when they are adsorbed together with CO, which can be an explanation for the fact that long-range alkali-CO interactions are essentially missing. Another result of the model calculations is that the energetic promotion effect is mainly a result of the charging of the alkali atoms whereas the lowering of the frequency is mainly a geometric effect.