Density functional calculations at the BP86/TZP level of theory are employed to examine the structures, binding energies, and vibrational frequencies of HCN-Ag-20 and CN--Ag-20 complexes Excited states are calculated using time-dependent density functional theory. A short time approximation for the perturbed density matrix and an empirical damping factor are employed to calculate surface-enhanced Raman scattering (SERS) intensities that account for both chemical and electromagnetic enhancement effects. Chemical effects provide 1-2 orders of magnitude enhancement, whereas electromagnetic effects yield up to 6 orders of magnitude enhancement for CN- or 5 for HCN. The structures and SERS enhancement factors suggest that multiple bonding configurations may be observed in experiment.