It is also customary to neglect contributions from vibrations. This is, in principle, a valid approximation when the interaction between adsorbate and substrate does not affect the vibrations significantly. Indeed, it has been shown that the vibrational entropy of the pristine substrate and the substrate in the combined monolayer/metal system tends to cancel. Conversely, the vibrations of the molecules on the substrate can differ substantially from the molecules in the gas phase, especially when charge-transfer at the interface occurs. For the special case of TCNE/Cu(111), it was shown in previous work the zero-point energy of the adsorption geometries differs only very little when there is no field applied.