[Pw_forum] Question on epsilon.x

[Andrea Ferretti]

Dear Vidur,

the "shift" parameter is used in epsilon.x to shift the transition 
energies (computed as differences of eigenvalues, thus "shift" works as a 
scissor operator). Imposing a non-zero shift value basically changes the 
origin of the frequency axis in the spectra.
If you do not have any special reason to use it (eg set empirically the 
DFT kohn-sham gap close to the experimental one, etc), just set it to zero
(or leave it out of the namelist)

Andrea

>
> I was doing some computations on dielectric functions, and noticed a
> 'shift' parameter that offsets the imaginary part of the dielectric function by some value.
>
> I wanted to know why is the parameter needed anyway, and how do we decide what value is used
>
> Vidur
>
> Vidur Parkash, Ph.D Candidate
> Electrical & Computer Engineering
> Michigan Technological University
> 716 EERC, 1400 Townsend Drive
> Houghton, MI-49931, USA
> Tel: 1-906-487-0023
>
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-- 
Andrea Ferretti
Oxford University, Department of Materials
Parks Road, Oxford OX1 3PH, UK
Tel: +44 (0)1865 612796;  Skype: andrea_ferretti
URL: http://quasiamore.mit.edu

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