  We apply the density-functional technique to determine the lattice constant
of GaAs supercells containing Mn_Ga, Mn_int, and As_Ga impurities, and use a
linear interpolation to describe the dependence of the lattice constant a of
Ga_{1-x}Mn_xAs on the concentrations of these impurities. The results of the
supercell calculations confirm that Mn_Ga does not contribute to the lattice
expansion. The increase of a is due to both Mn_int and As_Ga, that are both
created in the as-grown (Ga,Mn)As in proportion to x, and that are most
probably present in a remarkable amount also in the best annealed materials.
