Optical properties of semiconductors within the independent-quasiparticle approximation

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Abstract
We develop a gauge-invariant formalism for calculating the optical properties of solids within the random-phase approximation and the GW approach to electron bands (the independent-quasiparticle approximation). We show that, within the scissors-operator approximation, it is enough to shift rigidly the optical spectrum, calculated according to the density-functional theory–local-density approximation, to higher energies by the gap correction. Moreover, a simple formula is given valid beyond the scissors-operator approximation.