Electronic States in Organic Molecular Crystals

Abstract

Organic molecular crystals (OMC) (see, e.g., [1]) are specific in several respects. The basic feature distinguishing these solids from their standard inorganic counterparts is the fact that the main composition units are organic molecules which, in the condensed matter state, preserve their physical identity well. This is due to the following facts: • Atoms in the individual molecules are bound by forces which have mostly a covalent character. This is in contrast with the binding forces among the molecules, which are usually of the van der Waals type. The remarkable difference between the strengths of the covalent and dispersion forces implies that, in many respects, the molecules keep behaving as though they were in a gas phase even though they are well oriented and have a well-defined position in space (model of an oriented molecular gas). • As far as the repulsive forces between individual molecules do not allow overlap of electronic clouds in the molecular ground states, the ground state of such molecular crystals can usually be successfully built up of ground states of the individual molecules. This allows inter alia ascribing, in the crudest approximation, of electrons to their molecules, i.e., full neglecting the exchange of electrons between different molecules.