Dear Prof. Hütter,<div><br /></div><div>I quickly want to summarize the errors I've had in hope that you can provide some answers as to why these happen and how I can fix them. </div><div><br /></div><div>So I ran some calculations of a 1x4x3 anatase (101) slab with different vacuum thicknesses [a = 10 Å, b = 20-70 Å, c = 15 Å (XZ periodicity for wavelet solver)] and tried the following combinations of FFT libraries which resulting in the errors stated below:</div><div><br /></div><div>1) FFTSG + EXTENDED_FFT_LENGTHS: fails for b = 70 Å with an "Index to radix array not found"-error</div><div>2) FFTW3 + EXTENDED_FFT_LENGTHS: fails at every value of b with "the FFT in the x direction is not allowed; n01 dimension 175"-error</div><div>3) FFTW3: fails for b = 70 Å with an "Index to radix array not found"-error</div><div><br /></div><div>From what you've told me I plan on going with the FFTW3 library from now on, but it does still fail with a sufficiently large box when EXTENDED_FFT_LENGTHS is disabled, but enabling this option makes the problem even worse as then the calculation fails no matter the size of the unit cell... </div><div><br /></div><div>So my question is why does the FFTW3 library give an Index error when you stated that it does not have an upper limit and why then does enabling the EXTENDED_FFT_LENGTHS result in the other error stated above?</div><div><br /></div><div>Best regards,</div><div>Léon</div><div><br /></div><div class="gmail_quote"><div dir="auto" class="gmail_attr">On Wednesday, 4 October 2023 at 09:57:02 UTC+2 Léon Luntadila Lufungula wrote:<br/></div><blockquote class="gmail_quote" style="margin: 0 0 0 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">Dear Prof. Hütter,<div><br></div><div>Thanks for the suggestion I'll switch to FFTW3 then, but I do still have one question about using FFTW3. Why do I get the error
<span style="color:rgb(80,0,80)">"the FFT in the x direction is not allowed n01 dimension 175" when switching from FFTSG/EXTENDED to FFTW3/EXTENDED, while I get an "</span><span style="color:rgb(80,0,80)">Index to radix array not found.</span><span style="color:rgb(80,0,80)">" error when switching of EXTENDED_FFT_LENGHTS?</span></div><div><font color="#500050"><br></font></div><div><font color="#500050">Kind regards,</font></div><div><font color="#500050">Léon<br></font><div><br></div><div><br><br></div></div><div class="gmail_quote"><div dir="auto" class="gmail_attr">On Wednesday, 4 October 2023 at 09:35:36 UTC+2 Jürg Hutter wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hi
<br>
<br>there is an upper limit for the FFT length for the FFTSG library (1024).
<br>I would switch to FFTW3 where no such limit exists.
<br>
<br>regards
<br>JH
<br>
<br>________________________________________
<br>From: <a rel="nofollow">cp...@googlegroups.com</a> <<a rel="nofollow">cp...@googlegroups.com</a>> on behalf of Léon Luntadila Lufungula <<a rel="nofollow">Leon.luntad...@uantwerpen.be</a>>
<br>Sent: Tuesday, October 3, 2023 2:06 PM
<br>To: cp2k
<br>Subject: [CP2K:19284] FFT problems with large unit cell
<br>
<br>Dear all,
<br>
<br>I am running some tests with long chain adsorbates on a TiO2 slab and I'm running into some problems with the FFT libraries. I initially had the FFTSG set as preferred library, because the manual stated that "The use of PREFERRED_FFT_LIBRARY FFTSG is required" when using the WAVELET solver. However, I just read in this post<<a href="https://groups.google.com/g/cp2k/c/QEveWI7UqV0" rel="nofollow" target="_blank" data-saferedirecturl="https://www.google.com/url?hl=en-GB&q=https://groups.google.com/g/cp2k/c/QEveWI7UqV0&source=gmail&ust=1697014280863000&usg=AOvVaw36ztrLdvm4T4mY2YCTot2q">https://groups.google.com/g/cp2k/c/QEveWI7UqV0</a>> from Prof. Hütter that you can use the FFTW3 library in combination with the WAVELET solver and I already read several times that using this library is generally faster compared to the internal FFTSG one. I also already had to enable the EXTENDED_FFT_LENGTHS option for calculations with smaller (10x10x30) cells as I ran into some "Index to radix array not found."-errors.
<br>
<br>Now, with these long adsorbates I have a cell of around 10 x 15 x 60 Å and the combination of FFTSG/EXTENDED_FFT_LENGTHS again gives "Index to radix array not found."-errors, while switching to FFTW3/EXTENDED_FFT_LENGHTS says that "the FFT in the x direction is not allowed n01 dimension 175".
<br>
<br>So I have two questions:
<br>1) Is it possible to calculate such a large cell without getting these FFT errors or is it just too large?
<br>2) Should I switch to FFTW3/EXTENDED_FFT_LENGTHS to speed up my calculations or is it fine to continue with FFTSG/EXTENDED_FFT_LENGTHS?
<br>
<br>Hopefully someone can help me out with this problem.
<br>
<br>All the best,
<br>Léon
<br>
<br>
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