Phase diagrams for organic/metal interfaces: Significance of configurational and vibrational energies

Numerous properties of organic/metal interfaces strongly depend on their polymorphism. To computationally predict the thermodynamically most stable polymorph at finite temperature and pressure, the state-of-the-art method is ab initio thermodynamics. However, several approximations made within ab initio thermodynamics have been developed with small adsorbates in mind, which are comparable in size to a substrate unit cell and have no or only few internal degrees of freedom, and it is unclear how well these approximations translate to larger, more complex organic adsorbates. In this work, we study this question exemplarily for the model system of tetracyanoethylene on Cu(111), which shows a Blyholder-like charge transfer that in amount and shape depends sensitively on the structure of the organic adsorbate. In particular, we investigate how the phase boundaries between different stable polymorphs are affected by the (temperature-dependent) vibrational and entropy terms. Based on these investigations, we find that, while the configurational entropy is de facto negligible for organics, the vibrational free energy can shift phase boundaries by several tens of kelvins. Moreover, we discuss how the vibrational free energy can be approximated and why said approximations work well for organic/metal interfaces.