(asym) and 665 cm -1 (sym), and further modes involving the diamond-core breathing mode (749 cm -1 ), FAl-AlF stretch (405 cm -1 ), in-plane Al2 vs. O2 (821 cm -1 ), out-of-plane Al2 vs. O2 (401 cm - 1 ), rocking (244 cm -1 ) vs. flying (171 cm -1 ) vs. out-of-plane (95 cm -1 ) motions of Al2O2 vs. F2, and the anti-symmetric out-of-plane wobble of Al2F2 (225 cm -1 ). Note that gif-movie files for all modes are available in Supporting Information. previous computational studies at lower levels of theory. Furthermore, the coupled cluster method with such a large basis set is often considered the 'gold standard' of computational chemistry. Indeed, the strong coherence between the computed and observed IR frequencies reinforce this notion. For this reason, the coupled cluster data provide a good reference set to be able to compare density functional approximations with. Density functional results. A total of 100 density functional approximations (DFAs) were tested against the coupled cluster reference data. Note that the same conditions were used as were employed for coupled cluster: using the aug-cc-pVQZ basis in the gas phase and without relativistic corrections. Hence, we looked at the bond lengths and angles (see Table ), and frequencies.