Vibrational spectrum of (CO)2 on Cu(100): Quantum calculations with 18 coupled modes

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 CO-stretch fundamental is calculated at various temperatures. Both the temperature and coverage dependence of both the average CO-stretch frequency and the corresponding linewidth are consistent with experimentally observed trends.