[QE-users] Format of FFT grid for Potential

[Paolo Giannozzi]

On Sat, Oct 10, 2020 at 5:52 AM Shivesh Sivakumar <
shiveshsivakumar at gmail.com> wrote:

>
> For instance, my FFT grid has dimensions 40x54x180. If I understand
> correctly, this means:
> 1) The lattice constant along 'a' is divided into 40 points, along 'b' is
> divided into 54 points and so on..
>

in general, what is divided into 40, 54, 180 points are the three primitive
lattice vectors generating the lattice

2) There are a total of 388800 (40*54*180) points printed, which correspond
> to the potential at each point , starting with varying 'x' first, then 'y'
> and then 'z'. Meaning,
> the first 40 points in the output should be (0,0,0), (0.025,0,0) ...
> (1,0,0).
>

(1,0,0) would be point 41 but is not in the grid, because it is equal to
(0,0,0) (if your plot seems to miss something, that's why). Same for the
two other directions. Xcrysden should output data in the same order as in
FFT arrays: see point 6.3 of the developers' manual,
https://www.quantum-espresso.org/Doc/developer_man/developer_man.html

Paolo

The next 40 points should be                   (0,1,0), (0.025,1,0) ..
> (1,1,0).
>

Am I right in this regard? Any help would be much appreciated.
>
> Thanks,
> Shivesh Sivakumar
> University of Washington-Seattle
> WA-98105
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-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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