Physically, these results make intuitive sense. As the surface becomes increasingly covered, the frustrated translation parallel to the surface becomes increasingly constrained, leading to smalleramplitude motion and, thus, larger vibrational frequencies with less entropic contribution. Conversely, the weakened binding energy at higher coverages leads to a larger amplitude motion normal to the surface (desorption), thereby lowering the frequency. The trends from the fcc-NN model were consistent with those of atop-NN: relatively mildly affected vertical motions vibrational frequencies and a more noticeable coverage effect in horizontal motions in the higher coverage region.