Finally, vibrations of the molecule give rise to a dipole, which has a potential energy in the field (analogous to the adsorbate dipole, yet much smaller). As mentioned above, we find that the change in the zero-point energy of the vibrations is small regardless of the orientation of the TCNE molecule relative to the field. Therefore, for TCNE it is appropriate to continue using the ideal gas model for vibrational degrees of freedom, i.e., µ vib gas (p, T , F ) ≈ µ vib ideal (p, T ). Taken together, we model the chemical potential of the TCNE gas phase in the electric field F as µ gas (p, T , F ) ≈ E vac mol (F ) + µ trans ideal (p, T ) + µ rot ideal (p, T ) + µ vib ideal (p, T ) (