The 1-(3-methoxycarbonyl) propyl-1-phenyl-(6,6)C61 (also called PCBM) is a C60 derivative widely used as an electron-acceptor in organic solar cells. To date, all the infrared spectra reported are experimental, mainly because of the calculations needed to study these structures are highly time-consuming. In this report we address for the first time the infrared spectrum calculation of PCBM with Cs symmetry by using the PW91/dnp level as implemented in the Dmol3 code. In this calculation we have found two intense peaks in the IR spectrum, that agree fairly with the 1187 and 1787 cm-1 measured experimentally.