OPTICAL-PROPERTIES OF SEMICONDUCTORS WITHIN THE INDEPENDENT-QUASI-PARTICLE APPROXIMATION

We develop a gauge-invariant formalism for calculating the optical pro perties of solids within the random-phase approximation and the GW app roach to electron bands (the independent-quasiparticle approximation). We show that, within the scissors-operator approximation, it is enoug h to shift rigidly the optical spectrum, calculated according to the d ensity-functional theory-local-density approximation, to higher energi es by the gap correction. Moreover, a simple formula is given valid be yond the scissors-operator approximation.