Use of energy bandgap adjustment when simulating optical properties to maintain correct energy dissipation: Application to Ruddlesden–Popper oxysulfides

The use of density functional theory (DFT) to calculate the optical properties of materials frequently leads to a predicted energy bandgap that is narrower than that experimentally determined. When the energy bandgap is incorrectly evaluated by DFT, the resulting absorption coefficient must be adjusted to give the correct value, in particular in the vicinity of the energy bandgap. Recently, a method has been developed whereby the dielectric coefficient for a material calculated using DFT is blueshifted and its amplitude is scaled such that the scaled function satisfies the same moment sum rule as the unshifted dielectric coefficient. However, while the moment sum rule is a necessary condition for correctly predicting the optical properties, it is not a sufficient condition. In the present work, it is shown that this method of scaling the imaginary part of the dielectric coefficient is based on the fact that the optical conductivity obtained using the fluctuation–dissipation (FD) theorem can be shifted to adjust the energy bandgap. This assumes that the energy dispersion is accurately calculated using DFT, although the energy bandgap is shifted. This shift is taken into account simply by modifying the energy associated with inter-band transitions in an expression for the electron conductivity obtained using the FD theorem within an independent particle approximation. The effectiveness of this method is illustrated by predicting the optical properties of particulate oxysulfide photocatalysts recently shown to promote visible-light-driven overall water splitting.