Understanding why crystallization in strong magnetic fields can lead to new polymorphs requires methods to calculate the diamagnetic response of organic molecular crystals. We develop the calculation of the macroscopic diamagnetic susceptibility tensor, chi(cryst), for organic molecular crystals using periodic density functional methods. The crystal magnetic susceptibility tensor, chi(cryst), for all experimentally known polymorphs, and its molecular counterpart, chi(mol), are calculated for flexible pharmaceuticals such as carbamazepine, flufenamic acid, and chalcones, and rigid molecules, such as benzene, pyridine, acridine, anthracene, and coronene, whose molecular magnetic properties have been traditionally studied. A tensor addition method is developed to approximate the crystal diamagnetic susceptibility tensor, chi(cryst), from the molecular one, chi(mol), giving good agreement with those calculated directly using the more costly periodic density functional method for chi(cryst). The response of pharmaceutical molecules and crystals to magnetic fields, as embodied by chi(cryst), is largely determined by the packing in the crystal, as well as the molecular conformation. The anisotropy of chi(cryst) can vary considerably between polymorphs though the isotropic terms are fairly constant. The implications for developing a computational method for predicting whether crystallization in a magnetic field could produce a novel or different polymorph are discussed.