The global minimum model has an additional effect from the mixed-site occupancy. Like the adsorption energy, vibrational frequencies have a site-dependency, as illustrated in Figure . CO * adsorbed on atop site has a lower tilting motion frequency, but it has higher pendulum, desorbing, and stretching motion frequencies. Therefore, as the dominant occupancy changes from mainly hollow sites to mainly atop sites, it entails a monotonic increase in pendulum, desorbing, and stretching frequencies. However, the tilting motion frequency showed a decreasing trend in the low-coverage region and an increasing trend in the high-coverage region. This trend is a result of the compound effect of the initial frequency downshift due to the site conversion (hollow site bound CO * → atop site bound CO * ) followed by the frequency upshift due to the lateral repulsion at higher coverages.