[Pw_forum] fixing the angle.

Lorenzo Paulatto paulatto at sissa.it
Tue Nov 3 11:50:58 CET 2009

On Tue, November 3, 2009 11:38, Bipul Rakshit wrote:
>  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
>  ... the angle
>  deviate from 60 to 59.999 degree

It does not, at least not in the output configuration you have printed
above. If you notice, what's happened is that the cell basis vectors have
rescaled of the same amount, they have just shrink all together. If, from
the data above, you have measured a change in the cell angles you have to
buy a new pocket calculator.

>  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?

No, constrains are used for inter-atomic angles, not for the cell.

> Or is there some other option?

Yes, there is one. It does not work with bfgs, but you are using damp-w so
it should not be a problem. It is called cell_dofree, you will find the
documentation in the input format manual.

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

It is in the input format manual: Doc/INPUT_PW.txt or Doc/INPUT_PW.html,
it is toward the end of the file, you will have to scroll down a bit.

best regards

Lorenzo Paulatto
phone: +39 040 3787 511
skype: paulatz
www:   http://people.sissa.it/~paulatto/

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