  First-principles density-functional calculations have been performed for zinc
monochalcogenides with zinc-blende- and wurtzite-type structures. It is shown
that the local-density approximation underestimates the band gap, misplaces the
energy levels of the Zn-3d states, and overestimates the crystal-field
splitting energy. Without spinorbit coupling, the order of the states at the
top of VB is found to be normal for all the ZnX phases considered. Upon
inclusion of the spinorbit coupling in calculations, ZnO in zinc-blende- and
wurtzite-type phases become anomalous. It is shown that the Zn-3d electrons are
responsible for the anomalous order. The effective masses of electrons and
holes have been calculated and found that holes are much anisotropic and
heavier than the electrons in agreement with experimental findings. The typical
errors in calculated band gaps and related parameters originate from strong
Coulomb correlations, which are found to be highly significant in ZnO. The
LDA+U approach is found to correct the strong correlation of the Zn-3d
electrons, and thus improves the agreement with the experimentally established
location of the Zn-3d levels. Consequently, it increases significantly the
parameters underestimated in the pure LDA calculations.
