We report calculations of the vibrational frequencies of CO dimer on Cu(100) using the recently developed vibrational self-consistent field code. Eighteen modes are treated explicitly within a three-mode representation of the potential. Nine symmetry-distinct doublets are observed and the corresponding frequencies are computed. The thermally broadened spectrum of the GO-stretch fundamental is calculated at various temperatures. Both the temperature and coverage dependence of both the average GO-stretch frequency and the corresponding linewidth are consistent with experimentally observed trends.