[Pw_forum] question about FFT standard set by Quantum Espresso

[stefano de gironcoli]

the density has the dimensions of a density, both is real and reciprocal 
space.

rho(G) = 1/\Omega \int rho(r) exp(-iGr) d3r

rho(r) = sum_G rho(G) exp(+iGr)

\Omega being the unit cell volume.

stefano

On 26/07/2016 18:54, Yu,Yue wrote:
>
> Dear all,
>
> I encountered a problem when trying add a extra term rho_extra(G) into 
> hartree energy calculation (which is done by subroutine v_h in 
> PW/src/v_of_rho.f90). Here rho(G) means density of charge in G space, 
> which is generated from rho(r) by doing FFT.
>
> Thus, I need to transfer my known rho_extra(r) (extra charge 
> distribution needed) to corresponding rho(G) by doing FFT with hand, 
> the problem is, I have no idea with the standard of FFT set by Quantum 
> Espresso.
>
> It is important to find how to properly get the rho(G) from rho(r) in 
> the same way as QE do.
>
> For example, how does  Quantum Espresso set the normalization factor 
> for FFT in a supercelll with  volume=V? (It could be put into rho (G) 
> as 1/V,  or into rho(r) as 1/V, or into both rho(g) and rho(r) as 
> 1/sqrt(V)).
>
> I will appreciate if you can offer any help.
>
> Thanks a lot !
>
> Best,
>
> Yue Yu
>
> ========================================
>
> Yue Yu
>
> Ph. D student
>
> Department of Physics, University of Florida
>
>
>
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