  The electronic and optical properties of (Zn,Mg)(S,Se) wide-gap solid
solutions are studied using ab initio techniques and starting from the
previously determined atomistic structure of the alloy. Compositional disorder
is shown to close substantially the gap with respect to the predictions of the
virtual-crystal approximation. The bowing of the fundamental gap vs.
composition predicted by our calculations is in very good agreement with
experiments available for the Zn(S,Se) pseudo-binary alloy. At temperatures
typical for MBE growth, the quaternary alloy displays a rather large amount of
short-range order whose effect is to slightly but unmistakably open the gap.
Our results agree well with recent experimental data for the quaternary alloy.
