Band gap energies calculated by the method of promoting an electron will be reduced in comparison to those calculated by addition and subtraction by an amount equal to the binding energy of the exciton. If, following Mitas[27], we suppose the exciton is of the Mott-Wannier [113, 114] type, then its binding energy is given by [76],
As the simulation cells used here are relatively small, the exciton is
artificially localised. This can be taken account of in an
approximate manner by choosing 
in Eq.(
) equal to
the edge length of the simulation cell. For the n=2 simulation cell
used in these calculations, the exciton energy is estimated to be
0.1 eV for the whole simulation cell. This correction
has already been added to the band gap results shown in
Table . It cancels out of the band width
results in Table .
