A combined Monte Carlo and Hückel theory simulation of orientational ordering in C60 assemblies

Orientational ordering of C60 molecules within monolayer and multilayer islands is a regularly observed phenomenon in scanning tunnelling microscopy (STM) studies. Here we simulate the orientational ordering seen in STM images via a novel combination of Monte Carlo and Hückel theory methods and compare to experimental data. A measure of the repulsive interaction energy between two adjacent C60 molecules is pre-calculated by estimating and processing the electron density distribution between them. Many combinations of molecular orientations are considered to encompass all the detail of the molecular orbitals. Pre-calculated intermolecular interaction energies are inputted into a simulated C60 island. Here, the center position of each molecule is fixed, but the molecules are allowed to rotate freely around their centers. A minimum in the total island free energy is sought by sequentially picking molecules at random and rotating them according to their neighbours. Results show significant correlation with experimentally observed features in both mono and multilayered islands on a variety of different substrates