Binding energies and lattice dynamics. . In real clathrates, and in most of our predicted structures, the guest molecules have a significantly looser packing. However, the cavity volume calculation is imprecise, and guest packing coefficients are in some cases not well-defined. In channels or extended networks of interconnected pores, it is not possible to calculate a packing coefficient for the guest. We have therefore plotted results in Fig. -4 only for the majority of structures with well-defined guest packing coefficient, see the ESI for details. The distribution of guest packing coefficients is centered near 55%, but has a wider range than expected. That is, some apparently stable structures have cavities that seem too small or too large for its guest. Many of the structures however appear to have near ideal cavities for CO 2 and Xe.