[Pw_forum] problem in vc-relax convergence

[Prasenjit Ghosh]

It is quite natural if you optimize the cell parameter along the
z-direction also in your vc-relax. The system will try to minimize its
energy by interacting with the periodic images along z, i.e., tending to
form bulk. In order to do so, the value of the cell parameter will reduce
along z.

If you are interested in finding the lattice parameters in the plane of
your sheet, you should do a vc-relax calculation keeping the z fixed.

Since you have a 4x4 unit cell, you can also try to do manually by varying
in-plane lattice parameter, plot the total energy vs a and find the minima
of the curve.

With regards,
Prasenjit

On 29 August 2017 at 10:37, <l.eslami at srbiau.ac.ir> wrote:

> On 2017-08-28 11:40, Lorenzo Paulatto wrote:
> > Dear Leila,
> >
> > if the total energy keeps decreasing, that it is completely normal: it
> > means that you started from an energy saddle point. It may however
> > indicate that your initial crystal structure was wrong.
> >
> > kind regards
> >
> >
> > On 28/08/17 07:22, l.eslami at srbiau.ac.ir wrote:
> >> Hi all,
> >>
> >> We are trying to perform vc-relax calculation for a single layer ZnO
> >> sheet in the presence of an oxygen vacancy in a 4*4 suppercell. We
> >> have
> >> used espresso-5.4.0. with different psudopotentials such as GGA,
> >> NORMCONS and non relativistic Pseudopotentials: "O.pbe-mt_fhi.upf and
> >> Zn.pbe-mt_fhi.upf,  as well as GBRV pseudopotentials:
> >> “o_pbe_v1.2.uspp.F.UPF” and “zn_pbe_v1.uspp.F.UPF”. The calculations
> >> do
> >> not return any error. However, convergences of force and pressure do
> >> not
> >> occur even after long time running of the calculations. In fact, the
> >> code starts to decrease force and pressure but after a couple of
> >> relaxation steps they increase. I would be grateful if anyone could
> >> kindly give me any piece of advice to solve the problem.
> >> many thanks,
> >>
> >> Leila Eslami
> >> Department of physics,
> >> Islamic Azad University,
> >> Tehran, Iran
> >> _______________________________________________
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>
>
>
>
> Dear Lorenzo,
> thank you for your kind attention. I checked the total energy of the
> system and it keeps decreasing. However, there is another ambiguous
> point in the results. In fact, as our system is a sheet and we set 15
> Angstrom distance between subsequent layers in z-direction, I expect
> that the z-components of atomic positions do not change during relation
> process. But after a couple of relation steps this happens. To me it
> doesn't seem normal!!!!
>
> regards,
> Leila Eslami
>   Department of physics,
>   Islamic Azad University,
>   Tehran, Iran
>
> _______________________________________________
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> Pw_forum at pwscf.org
> http://pwscf.org/mailman/listinfo/pw_forum
>



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