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We study the electronic structure of the Mo 4d and W 5d impurities in GaAs, using the cluster approach within the multiple-scattering X\ensuremath{\alpha} formalism; results are also given for the Cr 3d isovalent impurity. Multiplet structures for the systems are evaluated through the approach of Fazzio, Caldas, and Zunger. Our results indicate that the Mo and W impurities will present a low-spin ground state in the 2+ oxidation state (acceptor), and that the observed luminescence lines are not related to the $^{5}\mathrm{}^{5}$${\mathrm{T}}_{2}$ internal transition of the isolated substitutional Mo and W impurities.

We present a comparison between two approaches\char22{}the linear combination of atomic orbitals\char21{}configuration interaction approach and the multiplet method developed by Fazzio, Caldas, and Zunger\char22{}to study excitation spectra of transition-metal impurities in a semiconductor. If we place the physically reasonable restriction on the configuration-interaction basis that only excitations between the highest (d-related) orbitals ${t}_{2}$ and e are allowed, we conclude that the results of both approaches are in excellent agreement.