[Pw_forum] vc-relax with BFGS

[Gabriele Sclauzero]

Il giorno 14/feb/2011, alle ore 14.18, Srijan Kumar Saha ha scritto:

> 
>   > Where is the proof that the final symmetry is different from the initial one? <
> 
>   Here goes more direct proof:
> 
>   Initial symmetry :  48 Sym.Ops. (with inversion). 
>   Final symmetry :  6 Sym.Ops. (no inversion). 

Where did you get this information from? Was it printed by the code? I never saw a printout of the "Final symmetry", but maybe I am wrong and the code computes and prints it. Which version of the code are you using?

>  
>  >  I think that you should not pay attention to differences of the order of 10^-9, <
> 
> If we compare the final & initial positions given in my previous email,  then we can see that
> the differences are quite large.

From your previous email it looks to me that the atoms are still in the original arrangement with cubic symmetry. I can guess that the volume of the cell has increased a bit (less than 1%) and so the atomic positions, which are always expressed in terms of the INITIAL celldm(1), will scale accordingly on the final output. The largest deviations I can spot out are below 10^-8.
Moreover, I don't understand why you want to perform such kind of expensive calculation on a supercell. Have you tried with the fundamental unit cell first?

GS


> These differences go away if we use 
> Wentzcovitch Damped Dynamics, instead of BFGS. However, Damped Dynamics stops    
> without convergence (50 iterations completed, stopping). Since this dynamics converge very 
> slowly, I would like to know whether VC-RELAX with BFGS has the above capability of respecting the 
> symmetry.
> 
> Thanks & regards,
> Srijan K.
> CCMT
> Indian Institute of Science
> India 560012 
> 
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§ Gabriele Sclauzero, EPFL SB ITP CSEA
   PH H2 462, Station 3, CH-1015 Lausanne

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