  We provide here a roadmap for modeling silicon nano-devices with one or two
group V donors (D). We discuss systems containing one or two electrons, that
is, D^0, D^-, D_2^+ and D_2^0 centers. The impact of different levels of
approximation is discussed. The most accurate instances -- for which we provide
quantitative results -- are within multivalley effective mass including the
central cell correction and a configuration interaction account of the
electron-electron correlations. We also derive insightful, yet less accurate,
analytical approximations and discuss their validity and limitations -- in
particular, for a donor pair, we discuss the single orbital LCAO method, the
Huckel approximation and the Hubbard model. Finally we discuss the connection
between these results and recent experiments on few dopant devices.
