The surface-enhanced Raman scattering (SERS) spectrum of pyridine adsorbed on Ag20 cluster (pyridine-Ag20) at room temperature is calculated by performing ab initio molecular dynamics simulations in connection with a Fourier transform of the polarizability autocorrelation function to investigate the static chemical enhancement behind the SERS spectrum. The five enhanced vibrational modes of pyridine, namely, υ6a, υ1, υ12, υ9a, and υ8a, can be assigned and identified by using a new analytical scheme, namely, single-frequency-pass filter, which is based on a Fourier transform filtering technique. To understand the factors evoking the enhancement in the SERS spectrum, the dynamic properties of molecular structures and charges for both of the free pyridine and adsorbed pyridine are analyzed. The calculated results indicate that the vibrational amplitudes of adsorbed pyridine are enhanced due to both of the electron transfer from pyridine to Ag20 cluster and the softening of pyridine bond. In addition, the N-Ag stretching within pyridine-Ag20 will couple with these five vibrational modes of pyridine. Consequently, the electron transfer between pyridine and Ag20 cluster induced by different molecular vibrational modes prompts the redistribution of electron density of pyridine. These factors collectively cause the noticeable change in polarizability during molecular vibrations and hence result in the enhancement of Raman peaks. © 2013 Wiley Periodicals, Inc.