[Pw_forum] Stress calculation in quantum espresso

[Lorenzo Paulatto]

In data 30 giugno 2009 alle ore 13:27:20, Huiqun Zhou <hqzhou at nju.edu.cn>  
ha scritto:
> BTW, there are other items need to check:
> (1) ion_dynamics
>  add 'bfgs' for the case of calculation = 'vc-relax'

This is already fixed in cvs, actually BFGS is the default case now, even  
for vc-relax.

> (2) pot | wfc_extrapolation
>  please clarify if 'first_order', 'second_order' can be used in  
> calculations  other than 'md' or 'vc-md'. I used these options in  
> 'relax' and 'vc-relax'  without any errors, too. Of course, I don't know  
> if these were really in
>  effect.

Both can be used for non-dynamical optimization (such as BFGS), although  
they don't striclty make very much sense. Nevertheless, they can help  
speeding up convergence, especially when the minimization steps get  
smaller. I would need to consult Paolo Giannozzi (who implemented the  
extrapolation) to be 100% sure.

> (3) ecfixed, qcutz and q2sigma
>  in the original paper, the framework is an extension of PR dynamics to
>  CP. Please clarify if these are effective in other dynamics (damp-w,  
> bfgs).  If possible, suggest a guide line how to set these options  
> (there is only  one example of carbon in the paper), say, qcutz should  
> be at least N (2?)  times of ecfixed, ecfixed should be ecutwfc plus  
> alpha (5 Ry or so?),
>  q2sigma should be one N-th of qcutz ...

They are used to compute a correction to the stress tensor, hence the  
apply to all kind of variable-cell processes (if it makes sense or not, I  
really cannot say).

regards

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Lorenzo Paulatto
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