(1) where i and k are atoms of type ι and κ belonging to molecules M and N respectively, separated by the distance R ik . The dispersion-repulsion force field parameters A ικ , B ικ and C ικ were taken from the W99rev6311P force field , supplemented with parameters for xenon . As is always done with the Williams force fields, the bond lengths to hydrogen atoms were shortened by 0.1 Å (69, 70). Electrostatic multipoles were calculated from the molecular charge densities with the GDMA program by Anthony Stone . Multipoles up to rank 4 (hexadecapoles) were distributed onto all atoms. Ewald summation was used for chargecharge, charge-dipole and dipole-dipole interactions, while dispersion-repulsion and interactions between higher multipoles were summed between whole molecules up to a centre of mass distance of 15 Å. By E latt we mean the sum of all intermolecular interactions per mole host molecule. The total energy of a whole simulation supercell with Z host molecules is ZE latt .