  Two-electron states bound to donors in silicon are important for both two
qubit gates and spin readout. We present a full configuration interaction
technique in the atomistic tight-binding basis to capture multi-electron
exchange and correlation effects taking into account the full bandstructure of
silicon and the atomic scale granularity of a nanoscale device. Excited
$s$-like states of $A_1$-symmetry are found to strongly influence the charging
energy of a negative donor centre. We apply the technique on sub-surface
dopants subjected to gate electric fields, and show that bound triplet states
appear in the spectrum as a result of decreased charging energy. The exchange
energy, obtained for the two-electron states in various confinement regimes,
may enable engineering electrical control of spins in donor-dot hybrid qubits.
