[Pw_forum] about example 03 (Al relaxation)

[Stefano de Gironcoli]

Aritz Leonardo wrote:

> Hello:
>
> I have a short question. I am trying to relax Magnesium surfaces in 
> the same way as example03 for aluminium.
>
> When I run the example what I don't understand is that only 
> Z-direction is relaxed. How come that atoms do not relax moving in X 
> and Y direction? As far as I understand from the input, there are not 
> fixed directions of atom positions so in principle they should all 
> move in 3 directions.

symmetry  (which is found by the code and preserved during relaxation) 
is the reason

stefano deGironcoli