[Pw_forum] geometry optimization

[mohnish pandey]

Thank you so much dear Lorenzo...

On Tue, Oct 27, 2009 at 9:32 PM, Lorenzo Paulatto <paulatto at sissa.it> wrote:

> In data 27 ottobre 2009 alle ore 15:00:25, mohnish pandey
> <mohnish.iitk at gmail.com> ha scritto:
> > I can fix this problem by preserving the symmetry and
> > getting eqb. lattice constan.?
>
> Dear Mohnish,
> the code actually do preserve the symmetry, it is enforced actively when
> computing the forces and cannot be broken during the relax procedure.
> What's happening is that the system has a lower symmetry than you think
> since the very beginning.
>
> There are two possible reasons:
> 1. you have not specified the initial atomic positions with sufficient
> precision, the code consider an atom to be in a high symmetry position if
> it is less than 1.d-6 away from it (I think in units of alat=celldm(0),
> but I would have to check)
> 2. the symmetry of your system has a free parameter you where not aware
> of, e.g. in a Wurzite structure the distance between alternate planes can
> change in a specific way without breaking symmetry.
>
> These two problems have two corresponding solutions:
> 1. specify more digits, or (with QE 4.1 and next) specify the position as
> a fraction, e.g.
>   Ga 1/3 2/3 0
> note that xcrysden cannot yet interpret an input file written like this
> and it is still a bit experimental
> 2. lock the atoms in position, you can do that adding three zeroes after
> the atom's coordinate in the input, e.g.:
>  Ga  0.3333333 0.6666667 0.000000  0 0 0
> have a look at the manual (Doc/INPUT_PW.txt) for details on details on
> this notation
>
> 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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-- 
Mohnish Pandey
Y6262,4th Year Undergraduate,
Department of Chemical Engineering,
IIT KANPUR
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