Ionic liquids from Car-Parrinello simulations, part I:: Liquid AlCl3

The properties of isolated AlCl3 clusters and the bulk system are investigated by means of static and dynamic electronic structure methods. We find important structural motifs with the edge connectivity dominant in a dimer and the corner connectivity dominant in a trimer. Furthermore, the trimer cluster exhibits an interesting ring structure with large cooperative effects relative to the dimer. Comparing the found structural motifs in isolated molecule calculations with the structure of the liquid allows us to determine the dominace of edge connectivity in the liquid. The size of the clusters present in the liquid indicates indeed that the dimer is the most abundant species, but there are also trimers, tetramers, and pentamers present. From the local dipole analysis both for the isolated clusters as well as for the liquid, further proof for the edge connectivity is given. However, all results point to the fact that there is also some small percentage of corner connectivity present that might be attributed to the most stable corner-connected cluster, namely the trimer. Importantly, we find that energetic considerations of isolated ( static) clusters only do not represent the findings in liquid phase. Instead, a quantum cluster equilibrium approach or simulations are needed.