[gpaw-users] get_eels_spectrum() - does this give the full "interacting" response function?

[Kirsten Andersen]

Hi Trevor,

The eels spectrum is calculated from macroscopic dielectric function,
which is obtained from the inverse dielectric matrix - this corresponds
to using the interacting density response function where local field
effects are included. See for example the tutorial at the gpaw homepage:
https://wiki.fysik.dtu.dk/gpaw/tutorials/dielectric_response/dielectric_response.html?highlight=df#gpaw.response.df.DielectricFunction

If I remember correctly the output returns a file with the loss spectrum
as two columns without and with local field effects. 

If you're only using RPA (and not ALDA) the Dyson equation is actually
not solved in the code. Instead the dielectric matrix is found from
chi0: 
epsilon = I - V  chi0
And then epsilon is inverted to account for local field effects. This is
equivalent to obtaining chi first and the using epsilon^-1 = I + V chi
as written in the method paper. 

Best regards
Kirsten



On Tue, 2014-11-18 at 19:14 +0000, Trevor Hardcastle wrote:
> Hello all,
> 
>  
> 
> I’d like to check that I properly understand what equations are
> actually being solved, and what steps are being taken analytically,
> when I command GPAW! to do the following:
> 
>  
> 
> from gpaw.response.df import DielectricFunction
> 
> df = DielectricFunction(calc='gs-diagonalised.gpw')    
> 
> df.get_eels_spectrum(xc=’RPA’)
> 
>  
> 
> Does  get_eels_spectrum() use the Dyson series expression to go beyond
> the Adler-Wiser formula for “chi0” and calculate the full
> “interacting” density response function “chi” using TDDFT? That is,
> does it follow the procedure summarised in equations (4) – (12) of Jan
> et al. PRB 83, 245122 (2011)?
>  http://journals.aps.org/prb/abstract/10.1103/PhysRevB.83.245122
> 
>  
> 
> Many thanks,
> 
> Trevor
> 
>  
> 
> ************************
> 
> Dr. Trevor P. Hardcastle 
> 
> EPSRC Doctoral Prize Fellow
> 
> Room B20.C
> 
> Engineering Building
> 
> Institute for Materials Research
> 
> University of Leeds 
> 
> LS2 9JT, United Kingdom
> 
> t.p.hardcastle at leeds.ac.uk
> 
> ************************
> 
>  
> 
> 
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