  We present ab initio calculations of total energies of Mn atoms in various
interstitial positions. The calculations are performed by the full-potential
linearized plane-wave method. The minimum energy is found for tetrahedral
T(As4) position, but the energy of the T(Ga4) site differs by only a few meV.
The T(Ga4) position becomes preferable in the p-type materials. In samples with
one substitutional and one interstitial Mn the Mn atoms tend to form close pair
with antiparallel magnetic moments. We also use the spin-splitting of the
valence band to estimate the exchange coupling Jpd for various positions of Mn.
It is the same for the substitutional and T(As4) position and it is only
slightly reduced for the T(Ga4) position. The hybridization of Mn d-states with
six next-nearest neighbors of the interstitial Mn explains the insensitivity of
Jpd to the position of Mn.
