  Molecules adsorbing on metal surfaces form a variety of different surface
polymorphs. How strongly this polymorphism affects interface properties is a
priori unknown. In this work we investigate how strongly the surface
polymorphism influences the interface work functions for various metal/organic
interfaces. To evaluate the whole bandwidth of possible polymorphs, we perform
full theoretical structure search, probing millions of polymorph candidates.
All of these candidates might be observed in reality, either by kinetic
trapping or by thermodynamic occupation. Employing first-principles
calculations and machine learning we predict and analyze the work function
changes for those millions of candidates for three physically distinct model
systems: the weakly interacting naphthalene on Cu(111), the strongly
interacting anthraquinone on Ag(111), and tetracyanoethylene, which undergoes a
re-orientation from lying to standing polymorphs on the Cu(111) surface. These
thorough investigations indicate that kinetic trapping of flat lying molecules
can lead to work function differences of a few hundred meV. If the molecules
also reorientate, this can increase to a change of several eV. We further show
that the spread in work function decreases when working in thermodynamic
equilibrium, but thermally occupied phases still lead to an intrinsic
uncertainty at elevated temperatures.
