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)