[Pw_forum] How/where to get DISPLACEMENT PATTERN

[Stefano Baroni]

Dear Prof. Stefano
I have taken the your suggestions for the first two of my questions.
Regarding the third one,
' 3. to do a 'vc-relax', what displacement pattern I better follow?'  
What I mean here is that in my in put file for the 'vc-relax' in what  
way I better displace(disturb) the atoms?
I have already done a phonon calculation for my system unrelaxed(the  
atoms in their ideal positions) and found negative phonon frequencies  
at gamma, M, and R.

--- On Sat, 1/31/09, Stefano Baroni <baroni at sissa.it> wrote:
From: Stefano Baroni <baroni at sissa.it>
Subject: Re: [Pw_forum] How/where to get DISPLACEMENT PATTERN
To: bayleyegn_aq at yahoo.com, "PWSCF Forum" <pw_forum at pwscf.org>
Date: Saturday, January 31, 2009, 4:53 AM

Dear Bayleyegn Akane:

> I am graduate student at Addis Ababa Univerity-Ethiopia. Currently I  
> am working on my thesis using the Quantum-Espresso package.

welcom on board!

>   I am trying to do phonon calculation with my system relaxed..  
> There are three points that  I want to be clear about for now:
> 1. when can I say that a system is fully relaxed?

when the forces acting on atoms and the acroscopic stress are "small  
enough". How small is "small"? As small as getting it even smaller  
would not change the quantity you want to calculate (phonon  
frequencies, in you case) to within your target accuracy. What the  
target accuracy has to be, only YOU can say ...

> 2. should I do simple 'relax' prior to a 'vc-relax'?

it depends how far your volume/ cell shape is from their equilibrium  
state. all in all, it is probably safer do some experimentation by  
yourself and gain some confidence on what you are doing, than relying  
on the advice of other people ...

> 3. to do a 'vc-relax', what displacement pattern I better follow?

sorry: I do not quite undererstand the question ...

take care -  Stefano B

---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
http://www.sissa.it/~baroni / [+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

Please, if possible, don't  send me MS Word or PowerPoint attachments
Why? See:  http://www.gnu.org/philosophy/no-word-attachments.html









---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
http://www.sissa.it/~baroni / [+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

Please, if possible, don't  send me MS Word or PowerPoint attachments
Why? See:  http://www.gnu.org/philosophy/no-word-attachments.html






-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20090131/bba575c7/attachment.html>