Abstract
Structure determination and prediction pose a major challenge to computational material science, demanding efficient global structure search techniques tailored to identify promising and relevant candidates. A major bottleneck is the fact that due to the many combinatorial possibilities, there are too many possible geometries to be sampled exhaustively. Here, an innovative computational approach to overcome this problem is presented that explores the potential energy landscape of commensurate organic/inorganic interfaces where the orientation and conformation of the molecules in the tightly packed layer is close to a favorable geometry adopted by isolated molecules on the surface. It is specifically designed to sample the energetically lowest lying structures, including the thermodynamic minimum, in order to survey the particularly rich and intricate polymorphism in such systems. The approach combines a systematic discretization of the configuration space, which leads to a huge reduction of the combinatorial possibilities with an efficient exploration of the potential energy surface inspired by the Basin-Hopping method. Interfacing the algorithm with first-principles calculations, the power and efficiency of this approach is demonstrated for the example of the organic molecule TCNE (tetracyanoethylene) on Au(111). For the pristine metal surface, the global minimum structure is found to be at variance with the geometry found by scanning tunneling microscopy. Rather, our results suggest the presence of surface adatoms or vacancies that are not imaged in the experiment.
Highlights
The specific structure of any material constitutes the key to its functionality
An innovative computational approach to overcome this problem is presented that explores the potential energy landscape of commensurate organic/inorganic interfaces where the orientation and conformation of the molecules in the tightly packed layer is close to a favorable geometry adopted by isolated molecules on the surface
The approach combines a systematic discretization of the configuration space, which leads to a huge reduction of the combinatorial possibilities with an efficient exploration of the potential energy surface inspired by the Basin-Hopping method
Summary
Dedicated to gas-phase structures or bulk crystals, recently, structure search was extended to organic/inorganic interfaces.[12,20−23] these methods often rely on elaborate data fitting or force fields to describe intermolecular interactions. The experimental STM shown ( ) in Figure 2b is only consistent with the 51 −1 6 unit cell (prompting us to neglect the others, because their calculations would only cost computational time but not provide more insight into the physics or the performance of our method) With this input, the assembly procedure generates approximately 200 000 different configurations. For all supramolecular polymorph calculations the experimental unit cell was measured from the representative STM image shown in Figure 2b with an epitaxy ( ) matrix of 5 −1
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