The quantification of lateral interactions on adsorbate enthalpy range from a simpler mean-field treatment using algebraic functions to a more rigorous spatially-resolved treatment using cluster expansion Hamiltonian . The effect of coverage on the entropy of an adsorbate has received far less attention, even though the coverage dependence of vibrational modes has been observed for decades . Detailed experimental information on low-frequency modes is difficult to obtain due to the detection limits, e.g. 200-4000 cm -1 for electron energy loss spectroscopy (EELS) and 1400-4000 cm -1 for IR ). Nonetheless, accurate determination of these large-amplitude, low-frequency modes is critical to the overall free energy, because these modes are responsible for the majority of the vibrational entropy. For example, the lowest vibrational mode for CO adsorbed on Pt(111) lies around ∼60 cm -1 , corresponding to frustrated translational motions . At elevated temperatures, a single harmonic oscillator with a frequency of 60 cm -1 makes a non-trivial contribution to the free energy, e.g. -0.14 eV at 200 • C and -0.25 eV at 600 • C.