[Wien] experimental vs relaxed parameters using wien2k

Mon Jul 31 10:07:14 CEST 2017

If you do a calculation then the lattice parameter(s) for the optimized structure is (are) correct for the used functional
as it results in the lowest energy, indeed only for this functional  (supposed you realy searched and found the absolute minimum for the correct magnetic order).
That's it.
Everything else is speculation.

If you used the wrong magnetic state in the calculation, then search for the correct one that gives the minimum energy.
If your experiment has different results, search which measurement was correct and which had a mistake.
Was somewhere just a typo ?
Was the assumed structure correct at all ?
If the data are from others, what are the details and purpose of the calculations or measurements ?
It is you that has to do the critical reading !

There exists no absolute thruth about the lattice parameters that someone can tell you.
If someone tells he can then he is a juggler.

I believe only in data that I faked by myself.

Ciao
Gerhard

DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy:
"I think the problem, to be quite honest with you,
is that you have never actually known what the question is."

====================================
Dr. Gerhard H. Fecher
Institut of Inorganic and Analytical Chemistry
Johannes Gutenberg - University
55099 Mainz
and
Max Planck Institute for Chemical Physics of Solids
01187 Dresden
________________________________________
Von: Wien [wien-bounces at zeus.theochem.tuwien.ac.at] im Auftrag von Chouaib AHMANI FERDI [ahmaniferdichouaib at gmail.com]
Gesendet: Freitag, 28. Juli 2017 10:49
An: A Mailing list for WIEN2k users
Betreff: Re: [Wien] experimental vs relaxed parameters using wien2k

Dear Wien2k users,

Thank you for your feedback.

If I got this right, I see that the optimization doesn't lead necessary to a good results and experimental data are there to evaluate the accuracy of the calculation method, and by that I mean the choice of parameters, potential, structure ...
The methodology depends also on the purpose of the calculation, and the properties we are looking for.

However, given the large interval in which reside the lattice parameters provided by different papers, how to be quite sure we have chosen the correct structure or that is closest to reality. I am aware of the fact that this is a vague question given the variety of the different compounds but is there at least some indications visible before or after the SCF cycles that could help evaluate the credibility of the .struct file ?

Faithfully,

[https://my-email-signature.link/signature.gif?u=139409&e=8861079&v=2883a1fe35cf8ce1aa0c1c472fead27ca9aa2e48be482983dfaf2d2a375c0d0e]

On Tue, Jul 25, 2017 at 4:55 PM, Tristan de Boer <tristan.deboer at usask.ca<mailto:tristan.deboer at usask.ca>> wrote:
You can find many papers that use either approach. I considered this somewhat tangentially in a paper on several semiconductor polymorphs that were synthesized under different pressures. [1] We collected x-ray absorption and emission spectra on thee polymorphs, and compared our spectra to those calculated using the PBE GGA functional for both relaxed and unrelaxed atom positions. We found that there was worse agreement with experiment using relaxed parameters compared to the experimental parameters, so at least for this case, using the experimental structure yields more correct results. I'm not sure how universal this is, and it may not apply as you move higher up the 'Jacob's ladder' of density functional approximations.
Best Regards,
Tristan

[1] doi:10.1103/PhysRevB.93.155205

On 2017-07-24 6:27 AM, Chouaib AHMANI FERDI wrote:
Greatings Wien users,

I just want to ask if the experimental parameters (lattice parameters,
internal parameter, atomic positions, ...) are already available for a
certain compound, would it be better to run SCF cycle with them or to
make preliminary relaxation and volume optimization and then perform SCF
cycle using the calculated parameters.

Faithfully,

--
AHMANI FERDI Chouaïb
Ph.D Student in Material Science
Ecole Normale Supérieure
Mohammed V University, Rabat.
Tel : +212 6 94 59 57 60

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--
AHMANI FERDI Chouaïb
"Laboratoire Matériaux Nanomatériaux Nanomagnétisme
  et Enseignement des Sciences"
Ecole Normale Supérieure
Université Mohammed V, Rabat.
Tel : +212 6 94 59 57 60