Before comparing the geometry optimization of different computational methods a reference method needs to be established. We employed the previously used set of 45 small dimer clusters found in Ref. 38. When choosing the reference method two concepts should be kept in mind, accuracy and computational wall time. When initially investigating the possible reference method on this small set of dimers a level of theory can be used which would be impossible to apply for the entire 1283 cluster data set described in Section 2.2.