Nonlocality and many-body effects in the optical properties of semiconductors

We report numerical calculations of the frequency-dependent dielectric function for different gauges of the electromagnetic held in the optical transition operator. Comparing the results, we draw conclusions about the importance of different nonlocality effects entering the calculations. Apart from the spatial inhomogeneity related to the atomic structure of matter, they are due to nonlocal pseudopotentials, quasiparticle self-energies, acid the incompleteness of the basis functions. Besides their influence on optical spectra, their effect on the validity of the f-sum rule and the magnitude of the resulting dielectric constants is also discussed. We present results for optical spectra where the many-body quasiparticle effect is included beyond the scissors-operator approximation. The group-IV materials Si, SiC, and C are considered as model substances.