  We use the density-functional calculations to investigate the compositional
dependence of the lattice constant of (Ga,Mn)As containing various native
defects. The lattice constant of perfect mixed crystals does not depend much on
the concentration of Mn. The lattice parameter increases if some Mn atoms
occupy interstitial positions. The same happens if As antisite defects are
present. A quantitative agreement with the observed compositional dependence is
obtained for materials close to a complete compensation due to these two
donors. The increase of the lattice constant of (Ga,Mn)As is correlated with
the degree of compensation: the materials with low compensation should have
lattice constants close to the lattice constant of GaAs crystal.
