Recently, I've come across discussions about the differences between fundamental (or electronic) bandgaps and optical bandgaps, with the two quantities differing mainly by the electron-hole binding energy (ref. this ResearchGate post and this Physics Stack Exchange post). A user made a comment in this post that "whether you are interested in the optical or fundamental gap, the KS DFT band gap should not be rigorously interpreted as giving either of those." Working with 2D materials where experimental bandgaps of monolayers are scarce, I often see DFT papers benchmark the calculated bandgaps with optical gaps (much like the ones described in this post). This leads to my question if there's a rule-of-thumb for how we benchmark DFT obtained gaps against types of experimental gaps (with considerations of balancing rigor and pragmatism)? In the same vein, how exactly should we think about DFT obtained gaps (i.e. conduction band minima minus valence band maxima from the bandstructure of an auxiliary noninteracting Kohn-Sham system) with respect to the bandgaps from experiments?
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