[CP2K-user] [CP2K:16404] Wrong Cell Parameters When Optimizing BaTiO3 Unit Cell

[Rylin Mao]

Dear Prof.  Kühne

Thank you for your reply!
I modified my code by using a diagonalization method with a 
4x4x4 Monkhorst-Pack k-point mesh. It indeed makes sense, however, there is 
still a big gap between the calculated cell parameters and the experimental 
results.
While using Cohen-Wu functional, the length of a and b axis seems 
acceptable, however, the length of c axis is about 4.6 angstroms. I think a 
difference of 0.6 angstroms is a little big. I also tried normal PBE 
functional. the calculated length of c axis is about 4.2 angstroms, still 
has a difference of about 0.2 angstrom from the experimental results. How 
can I improve my codes?
Looking forward to your reply!

Best regards,
Rylin Mao
在2021年12月22日星期三 UTC+8 00:40:36<tkuehne> 写道：

> Try to use at least a 4x4x4 k-point mesh. 
>
> Best, 
> Thomas Kühne
>
> Am 21.12.2021 um 16:46 schrieb 毛瑞麟 <ruilin... at gmail.com>:
>
> Dear CP2K Users:
>         I faced some problems while optimizing Tetragona BaTiO3 unit 
> cell.  I use direct cell optimization method, with Cohen-Wu Exchange 
> correlation functionals.  As described in literature, Cohen-Wu functional 
> is the best functional which can be used in ferroelctric systems. However, 
> the cell parameters after optimization EXTREMELY (a=b=4.30 angstroms, 
> c=5.59 angstroms) diverge from  cell parameters measured by experiments 
> (a=b=3.99 angstroms, c=4.04 angstroms). All electron steps converge 
> normally. 
>         I also tried normal PBE functional and PBEsol functional, the cell 
> parameters seemed far away from what is normal. 
>         So can anyone tell me what's wrong with my codes or how can I 
> modify my code?
> Best regards
>
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> <0.out><1.inp><3-1.cell>
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> ==============================
> Thomas D. Kühne
> Dynamics of Condensed Matter
> Chair of Theoretical Chemistry
> University of Paderborn
> Warburger Str. 100
> D-33098 Paderborn
> Germany
> thomas... at upb.de
> +49/(0)5251/60-5726 <+49%205251%20605726>
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