[Pw_forum] Imposed symmetry

[Gabriele Sclauzero]

Stefano Baroni wrote:
> 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. 

Please Stefano help me to understand this point, which is not clear to me. If the ground 
state is degenerate (you meant degenerate at a fixed external potential, i.e. fixed atomic 
geometry, am I right?) you could reach a lower energy by lowering the symmetry: do you 
think this is possible in practical calculations. I thought that you could only end up in 
a configuration with higher symmetry than the starting one, not lower.
 From the theoretical point of view, is it correct to apply the standard formulation of
HK theorem to a system with degenerate GS (is the HK mapping still valid?).

Gabriele

> 
> ---
> 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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