Guest insertion. We have investigated xenon atoms, methane and carbon dioxide as guest molecules. The guest molecules were inserted into crystal structures using a simple docking mechanism. The centre of mass of the guest molecule is placed at the centroid of the cavity, as determined with PLATON. For guests other than xenon, the molecule was then rotated randomly until there were no clashes between atoms, using 85% of the summed van der Waals radii as the criterion for a clash. Guest insertion was performed into the crystal structures in space group P 1, so that we did not restrict the positions and orientations of the guest to the symmetry of the original host structure. The guest docking was repeated until there were no more empty cavities in the unit cell. The structures were then relaxed by energy minimization, see below. The carbon dioxide molecules were supplemented with four non-interacting dummy atoms located around the carbon atom, since the program DMACRYS cannot handle linear molecules . The packing coefficient of the guest was calculated from the solvent-accessible void volumes, as calculated with PLA-TON, and the molecular van der Waals volumes: 25.85 Å 3 for CH 4 , 38.16 Å 3 for CO 2 and 42.21 Å 3 for Xe.