[CP2K-user] [CP2K:19299] Re: Cutoff for cells with different sizes

[Léon Luntadila Lufungula]

Dear Matt,

Thanks for the reply! 

Your answer already gives some good arguments about how to proceed, but 
I'll do some checks w.r.t. time saved with smaller boxes and the effect of 
the reduced PW basis on the energy to make sure I know what the effect of 
either option is.

All the best,
Léon

On Wednesday, 4 October 2023 at 07:56:20 UTC+2 Matt Watkins wrote:

> You should check. If your cutoff is large enough the small changes in the 
> number of grid points as you change box size should be small.
> OTOH you might not really be saving much compute time with the smaller 
> boxes so just having a constant box would avoid any possible errors from 
> that source. Typically Ti is quite heavy (many basis functions) and the 
> main cost will be in the SCF optimization of the density not in the PW 
> operations.
> Matt
>
> On Monday, 25 September 2023 at 11:02:24 UTC+1 Léon Luntadila Lufungula 
> wrote:
>
>> Dear all,
>>
>> I am doing calculations on a TiO2 slab modified with certain adsorbates 
>> and looking at the different conformations the adsorbates can take up when 
>> adsorbed onto the surface. For my calculations, I generally create a 
>> starting structure and then center the system and add a fixed amount (20 Å) 
>> of vacuum padding. I was wondering if this might pose any problems with 
>> respect to the plane wave basis set size as I know that the size of your PW 
>> basis set is dependent on both the cutoff energy and the volume of your 
>> simulation cell. Since some structures are less extended in the direction 
>> perpendicular to the slab, adding the vacuum padding will result in 
>> different volumes for each of the structures. Therefore I was wondering if 
>> this is a wrong procedure for me to follow, or if the differences would be 
>> only minute? I'm doing it in this way so as to minimize the cell volume and 
>> reduce the computational cost due to the plane waves. Alternatively, I 
>> would have to choose a large enough cell size that can fit all of my 
>> structures (+ the appropriate vacuum padding), but this would make some 
>> calculations unnecessarily heavy as the vacuum padding is much larger than 
>> required...
>>
>> Any suggestions would be much appreciated.
>>
>> All the best,
>> Léon
>>
>

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