For each species, molecular geometry optimizations were carried out in Gaussian 09 61 using the B3LYP hybrid functional and the 6-311G** Pople basis set. With this as a fixed molecular geometry, structures were generated using a quasi-random sampling method using the GLEE (Global Lattice Energy Explorer) code . Here each space group considered is sampled separately, by generating trial structures with unit cell dimensions, molecular positions and orientations sampled using a low discrepancy method. Approximately 10,000 valid (successfully lattice energy minimized) structures were generated in each of the 11 most common space groups (P 2 1 /c, P 2 1 2 1 2 1 , P 1, P 2 1 , P bca, C2/c, P na2 1 , Cc, P ca2 1 , C2, P 1) with one independent molecule in the crystallographic asymmetric unit (Z = 1). Approximately 85% of known molecular organic crystal structures are observed to crystallize with Z ≤ 1 in one of these space groups. For adenine, the known space group of form (II) (F dd2) was also included. All generated trial crystal structures were geometry-optimized with the molecular geometry kept fixed using the crystal structure modelling code DMACRYS . Intermolecular interactions were evaluated using the FIT+DMA anisotropic force field , which has the general form