Recently there has been a lot of interest in nanocrystals as nanosize particles embedded in a matrix and as independent particles in a powder. In some problems, one starts with the known nanocrystal size and calculates the adsorption energies or any other physical property. In other problems, the physical properties are first measured and from that the nanocrystal size is determined. The understanding of the physical properties of the nanocrystals is very important because of their application in the device fabrication. In the modern times, the components are often of the size of a few microns. If these components can be made within the size of a few nm, there will be a gain of a factor of 103.