Adsorption energetics, structural and vibrational properties of the Cu(001)-c(2 X 2)-CO system are studied with periodic slab calculations within density functional theory in the local density (LDA) and generalized gradient (GGA) approximations by using ultrasoft pseudopotentials. The recent expression of Perdew, Burke, and Ernzerhof (PBE) has been used as the GGA functional [Phys. Rev; Lett. 77, 3865 (1996)]. The fully relaxed geometry of the system is determined for different adsorption sites,of the CO molecule; unlike LDA, PBE-GGA predicts correctly the on-top adsorption site of the molecule. C-O and Cu-CO bond lengths, stretching frequencies, Cu-layer displacements, chemisorption energies, and work functions are computed under both approximations and are compared to the experimental values. LDA. gives accurate,values for the C-O equilibrium distance and the corresponding stretching frequency, while PBE-GGA better describes the adsorption bond and all the remaining properties.