[Pw_forum] Imposed symmetry

[Stefano Baroni]

Gabriele is totally right. Let me elaborate a little bit on his  
arguments, though.
Geometry optimization is based on energy minimization, which procedes  
by varying the structural parameters (nuclear coordinates, unit-cell  
shape/size) using various algorithms, all of which vary the struc  
parameters along energy derivatives (forces and stress). When the  
ground state is nondegenerate, energy derivatives have the same  
symmetry as the the geometry of the system. Energy minimization,  
therefore, cannot break any symmetry unless: i) random displacements  
are introduced on purpose (as suggested by Gabriele, and as sometimes  
done); ii) some noise comes in unporposedly, because of numerical  
accuracy (this is often coped is, again as suggested by Gabriele, by  
explicitly simmetrizing the resulting forces), or iii) if the ground  
state is degenerate, giving rise to Jahn-Teller distortions.
I do not expect to have been very pedagogic, but hope nevertheless to  
have offered some food for thoughts.
Stefano

On Feb 5, 2009, at 5:38 PM, Gabriele Sclauzero wrote:

>
> Green Power wrote:
>> Dear PWscf users,
>> I wonder if PWscf can deal with imposed symmetry for structural
>> relaxation calculations. If so, dose the imposed symmetry speed up  
>> the
>> calculations? Thanks in advance.
>
> The symmetry is determined at the beginning of the run and depends  
> on the bravais lattice,
> atomic positions and maybe other stuff (like starting magnetization  
> for noncolinear
> calculations with SO).
> Everything is symmetrizing with this initial symmetry which holds  
> during all the relaxation.
> You may want to break symmetry, rather than imposing it, when doing  
> structural relaxations
> in order not to be confined in a subspace of configurations.
>
> If you want to impose constraints, well, that's another story...
>
> GS
>
>
>>
>> Tian
>> Georgetown University
>> Washington DC 20057
>>
>>
>>
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>
> -- 
>
>
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
http://www.sissa.it/~baroni / [+39] 040 3787 406 (tel) -528 (fax) /  
stefanobaroni (skype)

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