[Pw_forum] small negative stress after optimization

[Stefano Baroni]

On Feb 25, 2012, at 1:47 AM, bahadir wrote:

> Hi
> 
> i get a small stress (0.31 KBar) after optimization of my system. I 
> could not be able to exact 0.0KBar for the optimization calculations. I 
> wonder if does it effect on other results such as band structure and 
> especially phonon calculations.

the short answer is NO. you can find a convincing explanation by yourself by calculating the volume variation that would correspond to such a small pressure (for this you will need the bulk modulus that you can either compute, or take from experiments, just for the sake of an order-of-magnitude estimate)

> Is it possible to get 0.00 KBar stress after optimization? My energy 
> threshold is 10E-14 and force threshold is 10-8.

of course, you will never obtain *exactly zero*. I believe that you can obtain an arbirtrarily small value for the stress, provided you have enough patience, computer time, and passion to obtain something that is not worth the effort ...

> thank you

you are most welcome. you will be even more welcome in the future if you stick to the current habit in this forum to sign the post you your name and affiliation.

SB

---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

La morale est une logique de l'action comme la logique est une morale de la pensée - Jean Piaget

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