[CP2K-user] [CP2K:18943] Re: k-points vs. supercell approach

[Léon Luntadila Lufungula]

Dear Matthias,

Thank you for the clarification, it does make sense when you put it like 
that. Could you perhaps clarify in what situation a wavefunction restart 
file might be available and how you would enable using this as a guess? I'm 
guessing you don't just mean when you restart an already finished or 
half-completed calculation, right?

Best regards,
Léon

On Tuesday, 20 June 2023 at 16:25:37 UTC+2 Krack Matthias wrote:

> Dear Léon
>
>  
>
> The timings appear reasonable to me. The run time for a small unit cell 
> with just 12 atoms using a moderate number of k points can be much faster 
> than a Gamma point calculation for a system with 600 atoms. The timings 
> might get a bit closer when a wavefunction restart file is available.
>
>  
>
> Best
>
>  
>
> Matthias
>
>  
>
> *From: *cp... at googlegroups.com <cp... at googlegroups.com> on behalf of Léon 
> Luntadila Lufungula <Leon.luntad... at uantwerpen.be>
> *Date: *Tuesday, 20 June 2023 at 15:09
> *To: *cp2k <cp... at googlegroups.com>
> *Subject: *[CP2K:18941] Re: k-points vs. supercell approach
>
> Dear all,
>
>  
>
> I am currently continuing with k-point sampling as this method is much 
> faster than the supercell calculations. However, I still don't quite 
> understand why/how this is the case... On most of the posts I read on this 
> forum, people state that the supercell approach should preferably be used 
> as this is the standard in CP2K, so I don't understand why there is such a 
> large difference in favor of k-point sampling in my case. Could anyone 
> confirm/deny that this could be normal behaviour? I still would prefer to 
> use the supercell approach as it has more functionalities, but I can't 
> consider it a valid option if the computational cost is so much larger than 
> the k-points approach...
>
>  
>
> Kind regards,
>
> Léon
>
> On Monday, 19 June 2023 at 14:18:48 UTC+2 Léon Luntadila Lufungula wrote:
>
> Dear CP2K community,
>
>  
>
> I'm quite new to CP2K and I am trying to build my input files step-by-step 
> to eventually do calculations on modified surfaces. Currently I'm still 
> doing single point calculations on my bulk structure (anatase TiO2) to 
> optimize the calculation parameters. I have already converged the values of 
> the CUTOFF and REL_CUTOFF parameters and was now looking at comparing 
> k-point sampling to gamma point calculations within the supercell approach. 
>
>  
>
> I was initially thinking about using the supercell approach as some things 
> are not yet implemented for k-points, such as the OT method for example, 
> but I noticed that supercell calculations are considerably slower than 
> k-point calculations. I have attached the input and output files for both 
> approaches for the largest cell/k-mesh I have used, which is 5x5x2. The 
> k-point calculation only took 14.342s, whereas the supercell calculation 
> took a whopping 649.442s...
>
>  
>
> So my question is if I should just continue with k-points sampling or if 
> there is something wrong with my inputs which results in the large 
> difference in calculation time? The k-point calculation also throws a 
> warning "*** WARNING in cryssym.F:165 :: Symmetry library SPGLIB not 
> available ***", but I also don't know if this may be the reason for the 
> discrepency or if I may ignore this warning.
>
>  
>
> Any help would be greatly appreciated!
>
>  
>
> Kind regards,
>
> Léon
>
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