ABSTRACT
The nanoclusters or nanostructures are intermediate between atomic and microscopic scales representing an emerging topic known as the mesoscopic physics, which is under intense research and in rapid growth. Applying the wave-particle duality to electron microscopy and neutron diffraction, we are able to probe atomic structure of crystals. An electron with the Bohr energy in a hydrogen atom could have a spatial extent of the Bohr radius. The possibility of contributions from other excited configurations of the atomic orbitals leads us to the configuration interaction model. To explore this regime of physics, this chapter begins by reviewing quantum mechanics. Theories of quantum chemistry generally utilize combinations of atomic orbitals to describe molecular structures. The construction of a molecular computer may however take two different routes: one along the line of semiconductor technology, the other the molecular biology. One of the ultimate purposes of nano computing is to be able to investigate complex molecules such as proteins for their structure.
