[Pw_forum] how to do fixed volume optimization

[Kun Yin]

Thanks Linh. I doubt that if I don't break the crystal symmetry, the 'relax' optimization of a supercell will have the same effect as the primitive cell. Even if not, there are two side effects of this idea: (1) more time to spend for computing a supercell, (2) difficulty to extract a primitive cell back from the supercell and appreciate its symmetry.

The 'vc-relax' calculation optimzes both the shape of the cell and atomic positions while keeping the symmetry at a target pressure. It takes the pressure as convergence criterion.

The 'relax'  calculation optimizes the atomic positions while keeping the shape of the cell. It takes the force on atoms as convergence criterion.

I want to optimize the both the cell parameters and atomic positions while keeping the symmetry and volume constant. It will take energy as convergence criterion.

I don't know if QE has the options in input to do this easily. This can be done by performing a series of 'relax' calculation to optimize all degrees of freedom in cell parameters and find the minimum value. But this is labrious especially for low symmetry structure such as monoclinic cell.
 
------------------				 
Kun Yin
2010-05-31

Kun Yin wrote:
> Thanks for your reply, Mohnish. 
> The 'relax' calculation will keep the volume constant but it will fix the cell parameters, too. For example, if I optimize a tetragonal cell,  I want the c and a axis to change but the volume unchanged. I know this could be done by varing c/a ratio manually with a series of relax calculation. My question is, can this be done automatically in QE
>   
Why don't you build a supercell that stores the considering structure.
When you do optimization, the volume of supercell will be fixed , but 
the structure is still optimized.

Hope this help.

Linh
> ------------------				 
> Kun Yin
> 2010-05-30
>
> Dear Yin 'vc-relax'  calculation optimizes the structures by the lattice
> parameters also but if you set the calculation 'relax' then it optimizes the
> geometry within the given fixed volume constraints. This calculation moves
> the atom keeping the volume constant within the given symmetry constraint.
> hope this helps :)
>
> On Sun, May 30, 2010 at 6:12 PM, Kun Yin <kyin.cn at gmail.com> wrote:
>
>   
>> Dear All,
>>
>> Does anyone have the experience to do volume fixed optimization with QE,
>> which means the structure is optimized allowing all cell parameters and
>> atomic positions to vary while keeping the volume fixed?
>>
>> Kun Yin
>> Najing University, China
>> 2010-05-30
>>
>> _______________________________________________
>> Pw_forum mailing list
>> Pw_forum at pwscf.org
>> http://www.democritos.it/mailman/listinfo/pw_forum
>>
>>     
>
>
>
>   


-- 
-----------------------------------------------------
 Nguyen Ngoc Linh, PhD Student
 c/o:   SISSA & CNR-INFM Democritos,
        via Beirut 2-4, 34014 Trieste (Italy)
 email: nnlinh at sissa.it
 phone: +39 04 03787 319
 skype: ngoclinh84phys
-----------------------------------------------------
"The physics is theoretical but the fun is real"


_______________________________________________
Pw_forum mailing list
Pw_forum at pwscf.org
http://www.democritos.it/mailman/listinfo/pw_forum