  We consider hypothetical Ga7MnAs8, Ga16MnAs16, and Ga14Mn3As16 crystals with
Mn in a substitutional, interstitial, and both positions. Spin-polarized FPLAPW
calculations were used to obtain their electronic structure. We show that the
interstitial Mn acts as a double donor and compensates the holes created by two
Mn atoms in substitutional positions. This explains why the number of holes in
Ga(1-x)Mn(x)As is much smaller than x. The presence of interstitial atoms may
also be the reason for the lattice expansion with increasing content of Mn. The
differences in electronic behavior of substitutional and interstitial Mn are
discussed.
