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[matthieu verstraete <matthieu.jean.verstraete@gmail.com>]

The executive summary:
sciss has nothing to do with GW calculations. The scissor operator is a purely ad-hoc way of fixing the electronic band gap to be close to that found experimentally, by shifting the (usually only conduction) bands rigidly. The GW calculation is a true ab initio prediction of the band gap of a solid.

On Sat, Feb 7, 2009 at 4:53 PM, Julie Smart <smart.julie@rocketmail.com> wrote:

Dear all,

I have gone through GW corrections for the band gap at gamma and I have made the convergency tests to go through calculating the imaginary part of dielectric function.

But when I read the description of  "sciss" I found that :

"Typical use is for response to electric field (rfelfd=3), but NOT for d/dk (rfelfd=2) and phonon responses. "

These are only for phonon and other DFPT response calculations: in some cases (esp for insulators) it is important that the electronic spectrum be qualitatively better than LDA can provide, eg to give a decent base dielectric function (epsilon_infinity) upon which to add the phonon-related low frequency behavior. Then, using the scissor is more acceptable, provided you know what the band gap should be, know you are actually interested in other quantities, and know the intrinsic errors you are making and limitations of the method - in particular the shift in bands is rigid, whereas the true correction to LDA bands is not a constant. Also, your spectrum no longer corresponds to the ground state you calculated. In some cases you even make an insulator out of a metal or semi-metal, and if you were to do so during the SCF run this would change things a lot.

The two concepts (scissor and GW) can be confused because they have a "similar" effect on the bands (scissor is made to mimic GW) and both allow you to get a more realistic dielectric function.

Matthieu

-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dr. Matthieu Verstraete

European Theoretical Spectroscopy Facility (ETSF) 
Dpto. Fisica de Materiales, 
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Mail : matthieu.jean.verstraete@gmail.com
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