Abstract
We compare the optical absorption of extended systems using the densitydensity and currentcurrent linear response functions calculated within manybody perturbation theory. The two approaches are formally equivalent for a finite momentum q of the external perturbation. At q=0, however, the equivalence is maintained only if a small q expansion of the densitydensity response function is used. Moreover, in practical calculations, this equivalence can be lost if one naively extends the strategies usually employed in the densitybased approach to the currentbased approach. Specifically, we discuss the use of a smearing parameter or of the quasiparticle lifetimes to describe the finite width of the spectral peaks and the inclusion of electronhole interaction. In those instances, we show that the incorrect definition of the velocity operator and the violation of the conductivity sum rule introduce unphysical features in the optical absorption spectra of three paradigmatic systems: silicon (semiconductor), copper (metal) and lithium fluoride (insulator). We then demonstrate how to correctly introduce lifetime effects and electronhole interactions within the currentbased approach.
Original language  English 

Article number  155203 
Journal  Physical Review B (Condensed Matter) 
Volume  95 
Issue number  15 
DOIs  
Publication status  Published  18 Apr 2017 
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Profiles

Myrta Grüning
 School of Mathematics and Physics  Senior Lecturer
 Atomistic Simulation Centre (ASC)
Person: Academic