Density functional theory (B3LYP/LANL2DZp) calculations on the structures and complex formation energies were performed to predict the alkali and alkaline earth ion selectivity of the cryptands 1,4,7,10,13,16,21,24-octaazabicyclo(8.8.8)hexacosane (N2N2N2) 1, 3,6,10,13,16,19-hexaazabicyclo(6.6.6)icosane (sarcophagine) 2 and 1,3,6,8,10,13,16,19-octaazabicyclo(6.6.6)icosane (sepulchrate) 3. Compounds 2 and 3 favour binding of Li⁺ and Mg²⁺ and exhibit a cavity size similar to [2.1.1], whereas cryptand 1 has a cavity size similar to [2.2.2] and prefers the selective binding of K⁺ and Ba²⁺. The cryptand flexibility of 1 is attributed mainly to the groups adjacent to the bridgehead nitrogens, whereas the main guiding feature for the flexibility of 2 and 3 can be traced back to the coordination of the ethylenediamine spacer.