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

[Matthias Krack]

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.

HTH

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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