The system, trans-1,2-bis (4-pyridyl) ethylene (BPE) on a Au substrate, is used to examine the applicability of cluster approximations for chemically enhanced molecule-surface Raman spectra. Different size of Au clusters, and adsorption sites, as well as different functionals were studied. In addition, the relative intensity of the two dominant modes of the theoretical spectra have been calculated for the different cluster sizes and adsorption sites. It was found that already a Au-20 cluster leads to excellent agreement with experimentally recorded spectra. (C) 2013 Elsevier B.V. All rights reserved.