# [CP2K-user] [CP2K:15725] PRESSURE TOLERANCE problems when doing cell opt with strain on 2D slab

Dragon Don shin... at gmail.com
Wed Jul 14 13:19:01 UTC 2021

```Dear  Thomas Kühne,

Thanks for your reply. Yes, you are right, This is an anisotropic system
with different hardness along x and y directions. Do you think it is the
anisotropy that leads to this issue? If so, what can I do to address it or
just avoid it.

My  best,
Dragon Don

2021年7月13日 上午6:49:00 (前天)


> Dear Dragon Don,
>
> is this an anisotropic system, e.g. a 2D system, with a different hardness
> along different directions?
>
> Best,
> Thomas Kühne
>
> Am 13.07.2021 um 00:49 schrieb Dragon Don <sh... at gmail.com>:
>
> 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
>
> 在2021年7月12日星期一 UTC+8 下午8:10:32<tkuehne> 写道：
>
>> 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
>>
>>   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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>> <QQ截图20210711202005.png><QQ截图20210711202109.png>
>>
>>
>>
>>
>> ==============================
>> Thomas D. Kühne
>> Dynamics of Condensed Matter
>> Chair of Theoretical Chemistry
>> Warburger Str. 100
>> Germany
>> tho... at upb.de
>> +49/(0)5251/60-5726 <+49%205251%20605726>
>>
>>
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