stretching factor in scissor operator

[Himani]

Dear sir,

What is the contribution of stretching factor for cbm and vbm in scissor operator for bethe salpeter calculation? Through scissor I am trying to give QP correction calculated through GW.
Can you please tell me what does it signify and whether its default value is ok when we are doing a BS calculation using double grid whose LDA vbm and cbm are at -0.6291 and 0.6291 respectively?
Please also tell me how to find it.

Regards,
Himani Mishra
Research Scholar
IIIT Allahabad

[Daniele Varsano]

Dear Hmani,
beside an energy shift to the gap, GW emerges with respect KS band structure can have a linear stretching of valence and conduction band.
The default values is just 1, which means a simple shift only.

The way to evaluate the stretching factor (if any) is to plot and fit linearly the QP energies with respect KS energies. They are column 3+ column 4, and column 3 in the o.qp output file.
You can use gnuplot or your favourite plotting software to visualise them and fit with a linear function and evaluate the slope. You can fit them valence and conduction separately by fitting positive and negative energies for valence and conduction respectively.

Best,

Daniele