[QE-users] Inconsistency of vc-relax output file

[Paolo Giannozzi]

Is the problem here the loss of the original symmetry of the lattice? if
hexagonal symmetry was present in the starting structure, it isn't lost at
the end (apart from pathological or unfortunate cases). If hexagonal
symmetry was not present in the starting structure, the final lattice may
no longer be hexagonal

Paolo


On Sat, Mar 23, 2019 at 7:05 AM Bharat Thapa <bhathapa at gmail.com> wrote:

> Dear developer,
>
>
>
> I have vc-relaxed Hf7O8N4 (hafnium oxynitride) assuming the crystal to be
> rhombohedra with a=b, alpha=beta=90 deg, and gamma=120 deg. So, I used
> ibrav=4 and gave A,B,C, cosAB, cosAC and cosBC values. Part of the input
> looks as follows:
>
>
>
> *&system*
>
> *    ibrav=  4                           *
>
> *    A=9.4851, B=9.4851, C=8.7867, cosAB=-0.5, cosAC=0.0, cosBC=0.0*
>
> *    nat=19  , ntyp= 3,*
>
> *    ecutwfc =70, ecutrho= 840, *
>
> *    occupations= 'fixed', *
>
> *    nbnd= 100 *
>
>
>
> The vc-relax ran successfully and I got the following output:
>
>
>
> *Final enthalpy =   -5322.7186767755 Ry*
>
> *Begin final coordinates*
>
> *     new unit-cell volume =   1536.19292 a.u.^3 (   227.64030 Ang^3 )*
>
>
>
> *CELL_PARAMETERS (alat= 17.92424134)*
>
> *   0.638651125   0.103631841  -0.047929083*
>
> *  -0.235833337   0.630643088  -0.049740867*
>
> *  -0.043220738  -0.085864416   0.635634338*
>
>
>
> Here, the output cell_parameter values are not consistent with the input.
> I assumed it as ibrav=4 (hex or rhombohedra), but if I use the output
> cell_parameter and calculated the final value (using the vector formulas
> for ibrav=4 from pw.x documentation) for lattice constant ‘a’, I will get
> different values from different vectors. So, my question is how do we
> analyse such a system in which we give certain crystal structure as an
> input and the vc-relax runs successfully (*does not crash*) and give an
> output which isn’t consistent with the input crystal system? Can I accept
> the results and carry on further because the vc-relax did not crash meaning
> that everything is fine?
>
>
>
> Please help me in this regard.
>
>
>
> Kind regards,
>
>
>
> Bharat Thapa
>
> PhD student
>
> UNSW
> _______________________________________________
> users mailing list
> users at lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/users



-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20190324/d3727544/attachment.html>