Yambo Community Forum

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

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