We report a calculation of the vibrational energies of CO/Cu(100) focusing on anharmonic coupling between the six CO-Cu modes, for Cu created as a rigid, multilayer slab. A realistic many-body potential [J. C. Tully, M. Gomez, and M. Head-Gordon, J. Vac. Sci. Technol. A 11, 1914 (1993)] is used to obtain a fourth-order force field in normal coordinates. The vibrational eigenvalue problem is solved using the vibrational self-consistent field method, and the fundamental frequencies are obtained for a thermal distribution of hot bands. The absorption spectra for the CO stretch, the CO-Cu stretch, the CO frustrated rotation, and the CO frustrated translation are calculated at two temperatures. All spectra are significantly broadened due to thermal effects of intermode coupling. Agreement with experiment is generally quite good.