Yambo Community Forum

Dear all

I've been looking through the YAMBO examples, most specifically the very useful quasi-particle band structure of a bulk material: h-BN exercise posted here:

http://www.yambo-code.org/wiki/index.ph ... rial:_h-BN

Towards the end of the exercise there's a section "Step 3: Interpolating Band Structures". Within the section there's a plot of the quasi-particle energy versus the LDA energy with a comment "we can see that the effect of the GW self energy is the opening of the gap and a linear stretching of the conduction/valence bands "

I appreciate this may be a "Devil's advocate" type of posting, but what I wanted to know is - physically - what's the significance of the stretching being linear, as opposed to presumably either zero or non-linear? What type of system would the stretching be expected to be non-linear?

I am guessing that the reasoning will be very well know to those who know, and I apologise for not being able to figure it out myself

With kindest thanks

Ian Shuttleworth
(School of Science and Technology, Nottingham Trent University, UK)

Dear Ian,
I cannot say if there is a physical meaning. Renormalization of the band velocity (ie a linear stretching beside the gap opening) is often observed in semiconductors.

What type of system would the stretching be expected to be non-linear?

Approximating GW correction by scissor+stretching does not apply in many cases, in general when you have a different degree of localization of orbitals or Bloch states, corrections turns to be very different: examples are usually molecules in general, defect states in semiconductors, metal complexes where d-bands corrections result very different from other levels corrections and surely many others.
Hopefully other users can provide a more physical explanation of the stretching.
Best,

Daniele