  First principle calculations based on LDA/GGA approximation for the exchange
functional underestimate the position of the semi core 3d levels in GaX (X = N,
P and As) semiconductors. A self-interaction correction scheme within the
LDA+U/GGA+U approximation is found to be sufficient to correct this
discrepancy. A consequence of thiscorrection is that the bandgap (E_g) of the
semiconductors also improves. The belief has been that the bandgap correction
comes from modified semi core-valence interaction. We examine this often used
approximation in great detail and find that although bandgap changes as large
as 0.63 eV for GaAs, 0.42 eV for GaP and 0.46 eV for GaN are obtained within
this approach for U= 20 eV on the Ga d states, only 0.1 eV, 0.1 eV and 0.15 eV
for GaAs, GaP and GaN arise from semi core-valence interaction. As U is
increased, the bandgap keeps improving. We trace this effect primarily to the
interaction of the Ga 4d states in the conduction band with the anion p states.
