The adsorption of a polyoxometalate (POM) on a metal surface has been studied by means of a periodic plane wave DFT based method for the first time. In particular, we have analysed the most favourable adsorption sites of anion [SiW₁₂O₄₀]^4- on the Ag(111) surface and computed the infrared spectrum. Despite the intrinsic complexity of the system we have been able to fairly reproduce the observed IR spectrum and unambiguously discern the signal corresponding to W-O-Ag symmetric stretching, which appears at around 800 cm^-1.