“…Therefore, the accuracy of the results will be very sensitive to the selection of the exchange-correlation potential (XC), which plays an important role in the determination of the energy gap. Based on our experience in using different XC potentials (LDA, PBE-GGA, EV-GGA, LDA-mBJ and PBE-GGA-mBJ) on several systems whose energy band gaps are known experimentally, [17][18][19] we nd that PBE-GGA-mBJ gives the best agreement with the experimental data. [17][18][19] This motivated us to use PBE-GGA-mBJ to calculate the band structure and hence the rst-order linear optical dielectric functions of I and II.…”