[QE-users] Format of FFT grid for Potential (Shivesh Sivakumar)

[Shivesh Sivakumar]

Hello,

I had previously taken a look at the pp.x documentation webpage, but
couldn't find information about the format in which the FFT is printed.
That is why I was wondering if my interpretation of it is correct.

Best,
Shivesh Sivakumar
University of Washington Seattle
WA-98105

On Sat, Oct 10, 2020 at 5:04 AM Marcelo Albuquerque <marcelofilho at id.uff.br>
wrote:

> Hi Shivesh Sivakumar,
>
> To understand how the grid is written in the file, you can check this
> page:
> https://www.quantum-espresso.org/Doc/INPUT_PP.html#idm167
>
> I hope it helps.
>
>
> Bests.
>
> *       Marcelo Albuquerque*
>
> *          Ph.D Student*
>
> *       Physics Institute*
>
> *Universidade Federal Fluminense (UFF)*
> *       Niterói/RJ - Brazil*
>
>
>
> On Sat, Oct 10, 2020 at 7:01 AM Shivesh Sivakumar wrote:
>
>
>> Hello users,
>>
>> I am trying to analyse the spatial variation of exchange-correlation
>> potential 'V' for my system. I am doing this using the 'pp.x' module in
>> the
>> directory containing the charge density data for my system. I also used
>> 'iflag=3' and 'output_format=5' which prints out a 3-D FFT grid which can
>> be readily plotted.
>>
>>  I am trying to understand the format in which this FFT grid can be read.
>> 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..
>> 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).
>>  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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