  Electronic structure of zinc blende AlN(1-x)$Px alloy has been calculated
from first principles. Structural optimisation has been performed within the
framework of LDA and the band-gaps calculated with the modified Becke-Jonson
(MBJLDA) method. Two approaches have been examined: the virtual crystal
approximation (VCA) and the supercell-based calculations (SC). The composition
dependence of the lattice parameter obtained from the SC obeys Vegard's law
whereas the volume optimisation in the VCA leads to an anomalous bowing of the
lattice constant. A strong correlation between the band-gaps and the structural
parameter in the VCA method has been observed. On the other hand, in the SC
method the supercell size and atoms arrangement (clustered vs. uniform) appear
to have a great influence on the computed band-gaps. In particular, an
anomalously big band-gap bowing has been found in the case of a clustered
configuration with relaxed geometry. Based on the performed tests and obtained
results some general features of MBJLDA are discussed and its performance for
similar systems predicted.
