[Pw_forum] fixing the angle.

[Bipul Rakshit]

Dear Lorenzo Paulatto and pwscf users

using vc-relax, i am doing simple LaSb (NaCl structure) with some lattice
parameter
and lattice vectors (CELL_PARAMETERS)

0.0 0.5 0.5
0.5 0.0 0.5
0.5 0.5 0.0

but after the relaxation
it converts to
0.0                0.4999999 0.499999
0.4999999  0.00             0.4999999
0.4999999  0.4999999  0.0

Although the change is very small, and when i draw the structure the angle
deviate from 60 to 59.999 degree
So if I just want to fix the angle exactly in 60 degree only or want to the
exact CELL_PARAMETERS as input, then how can i use the constraints flags?
Or is there some other option?

Actually there is no such documentation for using constraints.
please guide me in this reference.


On Mon, Nov 2, 2009 at 2:55 PM, Lorenzo Paulatto <paulatto at sissa.it> wrote:

>
> On Mon, November 2, 2009 10:13, Bipul Rakshit wrote:
> >  I saw the Doc/INPUT_PW.txt file, there is a flag called CARD:
> CONSTRAINTS
> >  in  which there is 'planar_angle' and  'torsional_angle', but it will be
> > more  helpful if you send me one input file for constraining the angle.
>
> I don't have any.
>
> >  Actually after running vc-relax with the input file which i send to you,
> >  the CELL_PARAMETER changes, that means angle also changing....
>
> Of course, if you squeeze the cell along one direction the structure will
> deform accordingly. If you want to rescale the whole structure maintaining
> all the angles constant you have to change ONLY celldm(1). There is no
> automatic way to do it, the cell_dofree input parameter can constrain cell
> deformation but not in this way.
>
> >  I draw the output file with xcrysden and find the angles (Si-O-Si)
> >  changes.
>
> Yes, I understand what's going on. But what's wrong with it changing in
> the first place?
>
> >  So if possible just send me some input file for constraining the angles.
>
> I don't have a secret library of input files...
>
> best regards
>
> --
> Lorenzo Paulatto
> SISSA  &  DEMOCRITOS (Trieste)
> phone: +39 040 3787 511
> skype: paulatz
> www:   http://people.sissa.it/~paulatto/<http://people.sissa.it/%7Epaulatto/>
>
>
>
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-- 
Dr. Bipul Rakshit
Research Associate,
S N Bose Centre for Basic Sciences,
Salt Lake,
Kolkata 700 098
India
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