The 'chemical mechanism' of surface enhanced Raman scattering (SERS) is investigated by quantum mechanical methods for pyridine adsorbed onto a copper cluster. Gaussian orbital based density functional theory with a B3LYP exchange-correlation functional is used to calculate the ground state structure and normal coordinates of the molecule-surface complex system, and the sum-over-states method, which uses excited state energies and dipole matrix elements from CIS (singles CI) calculations and the INDO/S semi-empirical method, is used to calculate the zero and non-zero frequency dependent polarizability derivatives that determine the Raman intensities. The cluster in these calculations is a copper tetramer whose excited state widths have been broadened to mimic interaction with bulk metal. The present method succeeds in describing the Raman spectrum of the adsorbed pyridine molecule, including changes in the spectrum that arise from adsorption on the surface, and differences between the zero frequency and finite frequency spectra. However the model is still quite primitive in its evaluation of the SERS enhancement factor.