A solid-state chemist's view of the crystal polymorphism of organic compounds.

In a survey of some structural and energetic aspects of crystal polymorphism, definitions are proposed, and a method for generating an unequivocal fingerprint of the cohesive pattern of an organic crystal structure is presented. The method identifies the electronic nature of molecule-molecule interactions in crystals, and its application requires a minimal training in basic crystallography and molecular modeling. The analysis suggests that thermodynamic and physical properties of polymorphs of organic crystals are quite often very similar, and sometimes depend on morphology as well as on crystal structure. It is also suggested that real polymorphism should be distinguished from the many defective or modulated structural variations often appearing in the crystallization of weakly bound molecular materials.