Our result allows an analysis of experimentally difficult-to-measure low-frequency modes. The six vibrational modes of CO * are visualized in Figure . The arrows in Figure aid in the visualization of the modes that move a direction tangential to the surface plane and modes that move in a direction normal to the surface plane. These six modes were grouped into the four motions by combining similar modes into one to facilitate an analysis. For horizontal motions (or motion in the plane parallel to the surface), the "tilting" motion is where the C atom is relatively stationary and the O atom is moving, while the "pendulum" motion is where the O atom is relatively stationary and the C atom is moving. These four modes are due to the simultaneous frustrated translation (diffusion) of the center of mass and frustrated rotation (libration) about the principal axes. The two vertical motions consist of a desorption mode and an internal stretching mode, which are characterized by changing bond length between Pt-CO and C-O, respectively. The coverage dependence of the vibrational frequencies for both systems is shown in Figure as average values over the number of CO * . By initially focusing our analysis on the atop-NN model's result of Pt(111) in Figure , the site-specific effect can be subtracted since all CO * are exclusively on atop sites. The changes in frequency from 1/9 ML to full coverage were +152cm -1 , +47cm -1 , -26cm -1 , and -45cm -1 for tilting, pendulum, desorbing, and stretching motions, re-spectively. The first observable trend is that the tilting motion is affected most significantly. Similar to the trend in the adsorption energy, both of the horizontal motion frequencies start to increase at 5/9 ML, which was precisely the same coverage at which adsorbate-adsorbate interaction (lateral repulsion) become increasingly repulsive. This result suggests that the horizontal motion frequencies are largely affected by the CO * -CO * lateral repulsion, whereas the vertical motions are less significantly influenced by the lateral repulsion. As a consequence of the increased lateral repulsion, the constricted movements in the horizontal direction increased their frequencies.