  We have investigated the electronic structures of various potentially
half-metallic Heusler compounds with the Tran-Blaha modified Becke-Johnson
(TB-mBJLDA) potential within the density functional theory. The half-metallic
gaps are considerably enhanced with respect to values from the
Perdew-Burke-Ernzerhof (PBE) functional. In particular the unoccupied densities
of states are modified by mBJLDA, and agreement with experiment is considerably
worse than for PBE. The agreement of the densities of states can be improved by
reducing the Tran-Blaha parameter c. However, ground state properties such as
the hyperfine fields are more accurately described by PBE than by mBJLDA.
Despite its success for ionic and covalent semiconductors and insulators, we
conclude that mBJLDA is not a suitable approximation for half-metallic Heusler
compounds.
