The energy differences between the polymorphs are very small and of the order of a few meV. Consequently, due to the differences in methodology, the best structure we find in this work (depicted in Figure ) is slightly different from the structure predicted in ref . We emphasize, however, that they differ only in the orientation of individual molecules. Both the coverage and the adsorption energy are virtually indistinguishable from the structure predicted to be the best in ref . A detailed comparison of the structures is given in the Supporting Information.