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<div class="moz-text-html" lang="x-unicode"> Dear all,<br>
<br>
I am trying to understand how Discrete Fourier Transform works in
QE (in particular fftw) and there are several questions for which
I would appreciate all kind of help.<br>
<br>
I have used as a guidance for learning the nice presentation
performed by prof. Giannozzi <br>
<a class="moz-txt-link-freetext"
href="http://www.fisica.uniud.it/%7Egiannozz/QE-Tutorial/tutorial_fft.pdf">http://www.fisica.uniud.it/%7Egiannozz/QE-Tutorial/tutorial_fft.pdf</a><br>
<br>
<br>
So if I understood correctly once the Ecutoff is set, the file
"gvectors.dat" contains the complete list of G vectors inside the
sphere with a radius 4*Ecut where magnitudes such like density can
be safely represented.<br>
<br>
<b>Starting from here, how is the real-space grid generated?</b> I
ask this because for my particular calculation there are around <b>3000
G different vectors</b> for a real space grid that has <b>nr1=nr2=nr3=24</b>
points.<br>
<br>
according to the definition:<br>
r= (i-1)/nr1+(j-1)/nr2+(k-1)/nr3<br>
<br>
The real space grid is denser than the reciprocal grid, so there
has to be some kind of mapping from one to each other that I am
missing. According to the above transparencies both grids should
have equal amount of points. <br>
<br>
I could still do (i think) brute force transformations using the
forward and inverse transformations defined in transparency 5 but
I if wanted to use fftw in order to be more efficient, shouldn't
they be the same in size?<br>
<br>
As I said, I would appreciate if somebody could address me a
reference or notes to read where these issues are explained.
Thanks! <br>
<br>
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