[QE-users] Format of FFT grid for Potential

[Shivesh Sivakumar]

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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