The electronic structures and magnetic properties of dilute magnetic
semiconductors (DMS) are investigated by using computational simulations
based on first-principles calculations and model calculations. Two
improvements in the simulation methods are proposed and implemented, in
order to discuss the following effects in DMS systems: i) the
self-interaction error in the LDA exchange-correlation functional and
ii) the inhomogeneous distribution of transition-metal atoms. To
overcome the self-interaction error in the LDA calculations, we have
developed a new scheme of the self-interaction-corrected local density
approximation (SIC-LDA) for KKR-CPA band structure calculations. The
density-of-states spectra calculated within SIC-LDA are generally in
good agreement with experimental photoemission spectroscopy data, while
the experimentally observed high-Curie-temperature in the wide band-gap
DMS such as (Ga,Mn)N and (Zn,Co)O is not expected from the SIC-LDA
results. One possible explanation for the discrepancy is the
inhomogeneous distribution of transition-metal atoms. For this problem,
the simulations of spinodal decomposition are performed. The results
show that the Curie temperature of wide band-gap DMS system can be
increased by making a connected cluster of transition-metal atoms, if
the impurity concentration is high ($15 \sim 20$ \%, for example).