  We investigate the thermal and electronic collective fluctuations that
contribute to the finite-temperature adsorption properties of flexible
adsorbates on surfaces on the example of the molecular switch azobenzene
C$_{12}$H$_{10}$N$_{2}$ on the Ag(111) surface. Using first-principles
molecular dynamics simulations we obtain the free energy of adsorption that
accurately accounts for entropic contributions, whereas the inclusion of
many-body dispersion interactions accounts for the electronic correlations that
govern the adsorbate binding. We find the adsorbate properties to be strongly
entropy-driven, as can be judged by a kinetic molecular desorption prefactor of
10$^{24}$ s$^{-1}$ that largely exceeds previously reported estimates. We
relate this effect to sizable fluctuations across structural and electronic
observables. Comparison of our calculations to temperature-programmed
desorption measurements demonstrates that finite-temperature effects play a
dominant role for flexible molecules in contact with polarizable surfaces, and
that recently developed first-principles methods offer an optimal tool to
reveal novel collective behavior in such complex systems.
