We present a methodology for the calculation of surface enhanced Raman scattering (SERS) spectra of molecules adsorbed on metal particles possibly immersed in a solvent. The molecule is treated at the ab initio level whereas the metal particle and the solvent are described through their dielectric properties. Harmonic vibrational frequencies are calculated analytically also including metal and solvent effects. Raman activities are obtained by differentiating with respect to normal coordinates the effective polarisability of the metal-molecule system. Enhancement factors and Raman spectra for pyridine adsorbed on complex-shaped silver particles are reported and discussed.