[Pw_forum] Question about vc-relax

[Hande Ustunel]

Dear Alejandro,

I think cell_dofree under the CELL section would do what you were looking
for. If you would only like to change the lengths of the cell vectors
without changing the angle inbetween them, you could use the option

    cell_dofree='xyz'

The internal parameter on the on the other hand would be spontaneously
relaxed during the geometric optimization to its optimal value obeying the
constrictions you have placed in your input.

Best regards,
Hande 

On Wed, 14 Jul 2010, "Alejandro Rébola" wrote:

> Hello everybody,
> 
> I'm trying to do a structural optimization for FeSe (tetragonal). I want
> pwscf to determine the optimal a and c and also one internal parameter:
> the z coordinate for Se. I'm trying to do a vc-relax calculation, but I
> get an error, apparently because I'm not including a Cell card. When I ran
> over the examples for vc-relax, a Cell card is included. However, in this
> case pwscf also changes the angles, and what I want is to keep the cell as
> tetragonal and only look for the optimal a, c and z. Does anybody know
> what I should do?
> Thank you in advance for your help.
> Best,
> 
> Alejandro
> 
> &control
>     calculation= "vc-relax",
>     pseudo_dir ='/home/alejandro/quantum_espresso/espresso-4.1.3/pseudo1',
> outdir='/scratch/alejandro/espresso'
>     verbosity = 'high',
>     prefix='FeSe',
>  /
>  &system
>     ibrav= 6,
>     a=3.7,
>     c=5.5,
>     nat= 4, ntyp= 2,
>     ecutwfc = 60.0, ecutrho = 350.0,
>     occupations='smearing', smearing='mp', degauss=0.01,
>  /
>  &electrons
>     mixing_beta = 0.5
>  /
>  &IONS
>    ion_dynamics      = "damp",
>    pot_extrapolation = "second_order",
>    wfc_extrapolation = "second_order",
>  /
> ATOMIC_SPECIES
>  Fe  1.  Fe.pbe-nd-rrkjus.UPF
>  Se  1.  Se.pbe-van.UPF
> ATOMIC_POSITIONS crystal
>  Fe 0.75  0.25  0.0  0  0  0
>  Fe 0.25  0.75  0.0  0  0  0
>  Se 0.25  0.25  0.2  0  0  1
>  Se 0.75  0.75 -0.2  0  0  1
> K_POINTS automatic
>  8 8 6 0 0 0
> 
> 
> 
> 
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-- 
Hande Toffoli
Department of Physics
Office 439
Middle East Technical University
Ankara 06531, Turkey
Tel : +90 312 210 3264
http://www.physics.metu.edu.tr/~hande