In the context of molecular based electronics, different molecular systems have been studied. In molecular crystals of pentacene, the conduction process has been reported to occur in a single layer in field-effect transistor applications. The electron localization length and its sensitivity to disorder of such two-dimensional disordered systems have been calculated. The electron localization in DNA has also been investigated. Our results show that DNA containing an approximately equal amount of the four basis is insulating. For instance, the human chromosome 22 is an insulator even though it has long-range correlation, which elsewhere has been suggested to give delocalized states.

The on-site Coulomb interaction has been calculated for rubidium-doped tetracyanoethylene (TCNE). If the electronic state is localized to a small region, e.g., a single molecule, the on-site Coulomb interaction becomes important. The interaction is a measure of the work needed to add an extra electron to the system. When the electronic state is localized to a single molecule/site it becomes large. The result has been used to interpret the outcome of ultraviolet photoelectron spectroscopy (UPS) measurements.