Energy model. Several of the methods and programs used have been described in detail elsewhere . The intermolecular (lattice) energy E latt is calculated with an atom-atom force field that includes electrostatic interactions between distributed multipoles derived from the molecule's charge density, as calculated with PBE0/6-311G(d,p). A polarizable continuum model (PCM) (67) with a relative permittivity of ε = 5.0 was used to model the polarization of the molecule in the crystal. All DFT calculations were performed with the GAUSSIAN 09 program . Intermolecular interactions between each pair of molecules M and N were modelled with an anisotropic force field of the exp-6 form, and with distributed electrostatic multipoles: