[CP2K-user] Two general questions about cell optimization in cp2k

Torstein Fjermestad tfjer... at gmail.com
Thu Apr 30 20:59:23 UTC 2020

Dear Matthias,

Thanks for your help. 

Concerning 1), is there any criterion for choosing the size of the 
reference cell?
Wouldn't it be necessary to do some kind of convergence test similarly to 
what we normally do for the energy cutoff?

Thanks and regards,

fredag 16. november 2018 15.28.17 UTC+1 skrev Matthias Krack følgende:
> Dear Torstein
> 1) CP2K provides the &CELL_REF 
> <https://manual.cp2k.org/cp2k-6_1-branch/CP2K_INPUT/FORCE_EVAL/SUBSYS/CELL/CELL_REF.html> 
> section in the &CELL section which allows for the definition of a larger 
> reference cell. The reference cell can be 5-10% larger. This results 
> basically in a larger cutoff value employed by CP2K. So you can also 
> request just a larger cutoff (and no reference cell) like with VASP.
> 2) I select usually the pressure from experiment. There is nothing special 
> about the default value of 100 bar. The value for the pressure_tolerance 
> <https://manual.cp2k.org/cp2k-6_1-branch/CP2K_INPUT/MOTION/CELL_OPT.html#PRESSURE_TOLERANCE> 
> is more interesting for cell optimizations. I employ a pressure tolerance 
> of 10 bar with DFT usually.
> Matthias
> On Wednesday, 14 November 2018 15:31:23 UTC+1, Torstein Fjermestad wrote:
>> Dear all, 
>> I have two general questions about optimization of cell parameters in 
>> cp2k.
>> 1) How can the Pulay stress be reduced in an optimization of the cell 
>> parameters?  In VASP, the recommendation  is to increase the kinetic 
>> cut-off energy by approximately 1.5 times the default kinetic cut-off 
>> energy. Are there any similar guidelines for cp2k?
>> 2) I see that the default value for the external pressure is set to 100 
>> bar. Why is this a reasonable default value? If we are comparing the 
>> computationally optimized cell parameters with the experimentally 
>> determined cell parameters, should we then change the value of the external 
>> pressure to the pressure for which the experiment was carried out?
>> Thanks for your help. 
>> Regards,
>> Torstein
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