  Electronic and nuclear spins of shallow donors in Silicon are attractive
candidates for qubits in quantum computer proposals. Shallow donor exchange
gates are frequently invoked to preform two-qubit operations in such proposals.
We study shallow donor electron properties in Si within the Kohn-Luttinger
envelope function approach, incorporating the full Bloch states of the six band
edges of Si conduction band, obtained from {\it ab initio} calculations within
the density-functional and pseudopotential frameworks. Inter-valley
interference between the conduction-band-edge states of Si leads to oscillatory
behavior in the charge distribution of one-electron bound states and in the
exchange coupling in two-electron states. The behavior in the donor electron
charge distribution is strongly influenced by interference from the plane-wave
and periodic parts of the Bloch functions. For two donors, oscillations in the
exchange coupling calculated within the Heitler-London (HL) approach are due to
the plane-wave parts of the Bloch functions alone, which are pinned to the
impurity sites. The robustness of this result is assessed by relaxing the phase
pinning to the donor sites. We introduce a more general theoretical scheme, the
floating-phase HL, from which the previously reported donor exchange
oscillatory behavior is qualitatively and quantitatively confirmed. The
floating-phase formalism provides a ``handle'' on how to theoretically
anticipate the occurrence of oscillatory behavior in electronic properties
associated with electron bound states in more general confining potentials,
such as in quantum dots.
