Fractionally charged ions in crystal lattices of organic ion-radical salts

The energy of an atom in a molecule is discussed on the basis of the definition of the chemical potential of electrons in a quantum system. An expression for the energy of the fractionally charged atom is proposed, and a master equation for the charge-dependent cohesive energy of a crystal is developed. Calculated net atomic charges in solid alkali halides are close to unity and their calculated cohesive energies indicate few percent of covalent bonding. The charge transfer at the energy minimum calculated for several organic, conducting ion-radical salts agree perfectly with the values determined experimentally. The lattice is found to be stabilized by the charge transfer, although the ionic interactions reproduce only part of the crystal cohesive energy.