While the properties of dopant impurities in Si are well understood for a very long time as a consequence of its invaluable role in the transistor industry, the details of the interaction between two donors are missing. These details do not impact traditional transistor technologies based on dilute doped semiconductors, but may be the key to novel few atom transistors. Moreover, the controllable interaction between donors is the fundamental tool to generate entanglement, the quintessential ingredient of quantum computation.