Investigating ordered phases of organic molecules with density-of-states simulations

Thin films of molecular semiconductors may show a range of ordered phases depending on properties of the molecules, of confining surfaces, and of the thermodynamic state. Computer simulations of such systems are challenging since the molecules are complex, the state space is multidimensional, and systems may be trapped in metastable states. To address these challenges we perform density-of-states simulations of a coarse-grained model in strategically chosen statistical ensembles. In recent work, we developed a model for alpha-oligothiophenes, where each molecule is represented as a linear chain of Gay-Berne disks. In this work, we perform Wang-Landau type simulations of the model to investigate crystalline and liquid crystalline phases in the bulk as well as ordering near surfaces. Simulations over a two-dimensional (volume-energy) state space allow us to reach very low energy conformations and analyze the bulk crystalline state. To investigate the structure of adsorbed layers under experimental conditions, we work with a revised state space and introduce confining fields. By combining simulations in different ensembles, we will follow the system from sub-monolayer coverage to bulk conditions.