[Pw_forum] problem with symmetry recognition

[Stefano de Gironcoli]

cell_dofree ?
i never used it but it may do something similar to what you look for.
stefano


On 07/09/2011 10:46 PM, giuseppe.mattioli at mlib.ism.cnr.it wrote:
> Dear Stefano, dear all.
> Ok, but suppose that I would like to put a (quite useful) constraint
> like: "I say that the lattice is cubic because I know that it MUST be
> cubic (or tetragonal or monoclinic or so...), than KEEP IT CUBIC while
> performing a vc-relax, irrespective of the stress tensor! Change the
> volume, change the a/c ratio (if the lattice is, e.g., tetragonal) but
> retain the space group. I often have to perform a (boring) multi
> dimensional fit by changing the lattice parameters by hand on a grid
> near to the "expected" minimum...
> I do not know exactly if Pascal's question was similar to mine;
> nevertheless I hope this can help...
>
> Yours
>
> Giuseppe
>
>
> Quoting Stefano de Gironcoli<degironc at sissa.it>:
>
>> this is not correct.
>> if the code finds some symmetry, forces and stress components will be
>> symmetrized accordingly  and vc-relax will keep it.
>>
>> There may be two reasons why the code does not find a symmetry:
>>
>> - the symmetry is not there... check your coordinates. Beware that you
>> need to specify ALL the atomic positions of a given structure not just
>> the inequivalent ones, hoping the code to fill the others.
>>
>> - the symmetry is there but involves a fractionary translations that
>> cannot be written as a multiple of the FFT grid spacing.  the code
>> writes a message saying that the symmetry is discarded due to a problem
>> with fractionary translation and says what the translation would be in
>> crystal coordinates. one can manually set the FFT dimensions (nr1,nr2,
>> and/or nr3) so that the translation is in register with the FFT grid
>> and the code can accept it
>>
>> HTH
>>
>> stefano
>> -
>> Stefano de Gironcoli - SISSA and DEMOCRITOS
>>
>>
>>
>> On 07/09/2011 07:58 PM, Vic Bermudez wrote:
>>> Hi Pascal,
>>>
>>> 	If I understand correctly, you're trying to optimize the unit cell while
>>> constraining 2 of the lattice constants to remain equal, as required by
>>> symmetry. I don't think that QE allows this. If you do a "vc-relax", what
>>> seems to happen is that symmetry is ignored, and all lattice constants are
>>> varied independently. I've attached a script written by a colleague that
>>> does repeated single-point runs for a range of lattice constants. You can
>>> then fit points near the total-energy minimum to find the exact minimum. Of
>>> course, this has to be done iteratively if more than one lattice constant is
>>> being optimized.
>>> 	Hope this helps.
>>>
>>> 	Best Wishes,
>>> 	Vic Bermudez
>>>
>>> Victor M. Bermudez
>>> Code 6876
>>> U.S. Naval Research Laboratory
>>> 4555 Overlook Ave., S.W.
>>> Washington, DC 20375-5347
>>>
>>>
>>>
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>
>
>
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