  The use of a special quasirandom structure (SQS) is a rational and efficient
way to approximate random alloys. A wide variety of physical properties of
metallic and semiconductor random alloys have been successfully estimated by a
combination of an SQS and density functional theory (DFT) calculation. Here, we
investigate the application of an SQS to the ionic multicomponent systems with
configurations of heterovalent ions, including point-charge lattices,
MgAl$_2$O$_4$ and ZnSnP$_2$. It is found that the physical properties do not
converge with the supercell size of the SQS. This is ascribed to the fact that
the correlation functions of long-range clusters larger than the period of the
supercell are not optimized in the SQS. However, we demonstrate that the
physical properties of the perfectly disordered structure can be estimated by
linear extrapolation using the inverse of the supercell size.
