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

[Bharat Thapa]

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20190323/b162b7ff/attachment.html>