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

[Bharat Thapa]

Dear Paolo,

Sorry for the late reply, however, I am wondering in this case, how do I
calculate the vc-relaxed lattice parameter? If it were cubic I would have
multiplied the output CELL_PARAMETER with the lattice and divided by 0.5.
But, as it is hexagonal (input) how do I figure out the relaxed lattice
parameters? Also, I want to know if the symmetry (hexagonal) has been lost.
How do I know the output file retains the input symmetry? The way I do is
by looking the the CELL_PARAMETER. But in this case the output
CELL_PARAMETER is not exactly representing that of ibrav= 4 but is somewhat
closer.
Please enlighten me on this; I am struggling a lot.

Thank you!

*Bharat Thapa*


On Mon, Mar 25, 2019 at 7:27 AM Paolo Giannozzi <p.giannozzi at gmail.com>
wrote:

> 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
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>
>
>
> --
> 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
>
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