Ab initio and Urey-Bradley calculations of normal modes were carried out to analyze the frequency shifts observed on the SERS spectrum of pyridine obtained on a Ag surface. A molecular model with a Ag atom attached to the pyridine nitrogen was used in the Urey-Bradley calculation. The frequency shifts in the SERS spectrum were calculated by adding K(Ag-N) and F(Ag...C-alpha) force constants and by adjusting the ring stretches according to how their bond lengths changed in an ab initio calculation with a (Ag-Py)(+) model. The ab initio calculations were done with the HF, MP2, and B3LYP methods using the 3-21 G,6-31G, 6-31G*, 6-31G**, and LanL2DZ basis sets. The surface active site was modeled in the calculation by means of Ag-0, Ag+ atoms, and a Ag-4(+) pyramidal cluster structure. The calculations were evaluated on how well they matched the observed frequencies, the nu(Ag-N) stretch, and the frequency shifts for the SERS spectrum in an electrochemical environment. The calculation with the Ag-4(+) model yielded the best results although the (Ag-Py)(+) model also gives reasonable results. All calculations were consistent with an edge-on interaction between pyridine and the electrode surface and a Ag+ species as part of the surface active site.