[Pw_forum] Lattice constant optimize

[Eyvaz Isaev]

Dear wangqj (sounds strange, right?) 

--- On Fri, 8/22/08, wangqj1 <wangqj1 at 126.com> wrote:
 
> Dear pwscf users
> I want to optimize the lattice constant of ZnO use
> VC-relax,should I set the X-Iforce,Y-iforce,Z-iforce to  0
> or 1 ? In my opinion ,I think they should be set > to 0. does it right ? 

No, it is not correct. If you mean like "Atom X Y Z 1 0 0", it means move atom in X direction, but keep fixed Y and Z coordinates.
Besides I do not remember keywords like X-Iforce etc., you can not fix forces, they are calculated self-consistently. For the ground (equilibrium) state, of course, they should be around zero (sometime forces are zeroes due to symmetry reasons).
If you just specify atomic position "X Y Z", so, atoms are allowed to relax their positions along X, Y, Z directions.
 
> besides, I want to know the reliability of
> this method. What's the criterion to know whether the
> derived lattice constant (a ,c/a)is right or not ?

Sorry, this one is your own responsibility to decide whether you got good parameters. See von Barth paper 
Physica Scripta. Vol. T109, 9­39, 2004 to understand basics of the DFT.
Compare with experiment and previous calculations and analyze why you got different parameters. More general, trial and fail method is very good. 

> Any useful advice will be appreciated .
>  
And we will appreciate providing your affiliation. And your own name.

Bests,
Eyvaz.

>       Sincerely
>  
>  
>  wangqj_______________________________________________

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Prof. Eyvaz Isaev, 
Theoretical Physics Department, Moscow State Institute of Steel & Alloys, Russia, 
Department of Physics, Chemistry, and Biology (IFM), Linkoping University, Sweden 
Condensed Matter Theory Group, Uppsala University, Sweden 
Eyvaz.Isaev at fysik.uu.se, isaev at ifm.liu.se, eyvaz_isaev at yahoo.com