The interaction of propyne on Cu(111) has been studied by periodic and cluster model density functional based methods. The structure of adsorbed propyne predicted by the different models and methods is almost indistinguishable and in very good agreement with experiment. This is a highly distorted propyne with C-1 and C-2 in nearly sp(2) hybridization. The adsorption site predicted by both surface models is also in agreement with experiment provided the cluster innermost region has the correct environment. The use of different functionals does not appear to have a noticeable effect on the adsorption geometry and position of the adsorbate relative to the surface. However, the adsorption energies computed by different exchange-correlation functionals may differ in roughly 0.35 eV. Periodic calculations carried out for different unit cells confirms the experimental long-range structure. Finally, a comparison of cluster and periodic adsorption energies permits to establish that the influence of the surface model on the predicted adsorption energies is much larger than the incertitude due to the choice of a particular exchange correlation functional.