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, we suppose the exciton is of the Mott-Wannier [113, 114] type, then its binding energy is given by ,
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 .