[Pw_forum] vc-md question

[Miguel Martínez Canales]

Hi again,

First, I'd like to thank you for your answers. Now, a couple of things

Paolo Giannozzi wrote:
>>2) On another ground, why are the stresses shown in a scf run and 
>>in a vc-md run different?
> 
> they shouldn't be (and I am quite sure they aren't). Are you by any 
> chance using the modified kinetic energy functional in vc-md runs?

No, I am not using it. You can see my input on my firt e-mail (I did a copy
and paste). However, I was wrong on this one. If I use all the decimal
places, I get stresses right: very similar when doing a vc-md after a scf,
and differing by about 0.4% when using the last vc-md output figures on a
scf run.

>>4) VASP featured fixing some atoms. As far as I know, espresso 
>>doesn't allow this (at least for pw.x runs). Am I totally wrong?
> 
> don't be so pessimistic: you are wrong on this specific point only :-)
> See the definition of ATOMIC_POSITIONS , variables if_pos.

Thank you very much, Paolo. I misunderstood that if_pos only applied to neb
and smd runs.

> I am seriously considering removing it altogether from future releases.

 From pw only or from all codes? Is there another simple way of performing
structural relaxation of noncubic structures? Especially with
non-orthogonal cells at high pressure.



To Guido Roma:

Yes, I know that the default limit is 50 iterations and that I am hitting
this limit. What puzzles me is that, after using the last vc-md coordinates
in a re-run, it still doesn't finish in 50 iterations. If I repeat the
process it will happen again.

For this simple tetragonal structure it would be more efficient for me to
see the stress at different c/a ratios and interpolate to achieve the
desired result (as Paolo suggested), but I won't be facing tetragonal (or
even orthorrombic structures) always.

Thanks again to everybody,

Miguel


-- 
----------------------------------------
Miguel Martínez Canales
    Dto. Física de la Materia Condensada
    UPV/EHU
    Facultad de Ciencia y Tecnología
    Apdo. 644
    48080 Bilbao (Spain)
Fax:  +34 94 601 3500
Tlf:  +34 94 601 5437
----------------------------------------

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Annonymous