k-points vs. supercell approach

[Léon Luntadila Lufungula]

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

[Léon Luntadila Lufungula]

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

[Krack Matthias]

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

 

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[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

[Krack Matthias]

I mean when SCF_GUESS restart becomes effective, e.g. the timings for the consecutive steps in an MD or CELL_OPT run.

 

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[Léon Luntadila Lufungula]

Okay, that's good to know as I'm currently running CELL_OPT calculations to evaluate my computational method/setup. I'm currently running them with k-points but I'll be sure to compare the timings with a supercell + SCF_GUESS RESTART approach.

Thanks a lot for the suggestions!