# [CP2K-user] [CP2K:15715] PRESSURE TOLERANCE problems when doing cell opt with strain on 2D slab

Dragon Don shin... at gmail.com
Mon Jul 12 22:49:00 UTC 2021

```Dear Thomas Kühne

I really appreciate your timely help. It's glad to know that the pressure
deviation is not that far off but why, I thought 1300 bar（=13 MPa）is quite
huge.
Indeed, I have tested another 400 optimization steps, but the pressure
deviation changes slightly, just fluctuating around 1300 bar.
It's strange that such convergence issue only happens when the strain is
loaded along the y axis (My cell is oblique hexagon as shown below).
And the larger the y strain, the further the pressure deviation reaches,
even up to 3000 bar for twice the y strain.
In contrast, the pressure deviation can easily converge to 100 bar for x
strain loading. The only difference is there are three cell parameters
(a,b,γ) that should be settled in y strain loading, while only b and γ
need to be calculated for x strain loading. So, Thomas, what can I do to

Dragon Don

> Dear Dragon Don?,
>
> converging the pressure up to 100 bar is a though call and -1345 bar not
> that far off.
> If, however, you are not satisfied by the deviation you simply need to
> need optimize
> for longer. But, be aware that the stress tensor sensitively depends on
> convergence criterium, so a tight convergence also there is critical!
>
> Best,
> Thomas Kühne
>
> Am 11.07.2021 um 14:24 schrieb Dragon Don <sh... at gmail.com>:
>
> Hi everyone！
>
>   Now, I am doing a cell opt with strain by fixing the cell size, while
> there is a problem that the pressure tolerance can not converge. Could you
> please give me a hand, looking forward for your help!
>
>   ps. The pics below are the part of the input and output,respectively.
>
>  Don<QQ截图20210711202005.png><QQ截图20210711202109.png>
>
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>
> ==============================
> Thomas D. Kühne
> Dynamics of Condensed Matter
> Chair of Theoretical Chemistry
> University of Paderborn
> Warburger Str. 100