[Wien] experimental vs relaxed parameters using wien2k

Tue Jul 25 14:36:56 CEST 2017

What better result do you expect ?

The Pearson data base has 99 entries for the same structure of ZnFe2O4  (as well as some others)
the lattice parameter spreads from 8.1758 to 8.5186 A with a mean of  8.42 +- 0.06 A (some are most probably for different temperatures)
the free u parameter is reported to be at least  in the order of 0.24 to 0.26 (I did not check all)

The structure is reported by Pearson as (MgAl2O4, cF56, F d-3m (227)) with disorder between Zn and Fe on the 8a and 16d sites (up to 0.5 Fe 0.5 Zn on one site !)
and the reported amounts of disorder (site occupations) are different for the various reported data.
To simmulate the antisite disorder with Wien2k is not straight forward, how did you manage that, did you use the ATAT package ?

Are you sure that the experimental lattice parameter you compare to is for very low temperature
and from a stoichiometric sample ? (various off-stoichiomnetric compounds close to 1:2:4 eg.: 0.98:2.02:4 or 0.97:2:4 as well as others are also reported)

btw: Schiessl W. reported in PRB for a=8.4599 A u=0.2405 at 300 K (the reported Zn deficiency is not mentioned in Pearson !!)

As I told, you should take care what you are calculating otherwise you compare apples with pears.

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: Dienstag, 25. Juli 2017 12:49
An: A Mailing list for WIEN2k users
Betreff: Re: [Wien] experimental vs relaxed parameters using wien2k

Thank you for your response.

If experimental data is available, I believe from what it has been said it is better to use them, otherwise, one have to perform optimization.

I did one myself on ZnFe2O4 and found a value of the cubic lattice parameter "a" not so close to experiment. also the internal parameter was not that near.

Exp :               a = 8.4599 A and u = 0.2563
optimized :    a = 8.2713 A and u = 0.2642     (more distortion of tetrahedra and octahedra)

(GGA-PBE), rkm = 6.5, k-points = 40

Is there any method that could yield better results ?

Faithfully,

[https://my-email-signature.link/signature.gif?u=139409&e=8721404&v=312a86cae413f6e92fc28afead97e850687cd9ee8ee825f076a7bfae0a649f3b]

On Mon, Jul 24, 2017 at 2:09 PM, Fecher, Gerhard <fecher at uni-mainz.de<mailto:fecher at uni-mainz.de>> wrote:
I would first check if the structure was realy completely solved in previous work.
I found that e.g. in the Pearson's crystal data base the structural parameters (atomic positions) for the prototype are reported for many compounds.
If one uses these parameters (and they do not accidentially fit the real ones) then the electronic structure will be most probably not correct,
this concerns also all other parameters (e.g. magnetic moments, optical spectra, etc.)
Indeed you do not need to optimize positions if they are fixed by symmetry.

Therefore my advice is, if the structure was not determined by yourself (or in your group) you should optimize it first before thinking about the calculated properties.
In some cases you may find afterwards that the structure initially thought about might be wrong.

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<mailto:wien-bounces at zeus.theochem.tuwien.ac.at>] im Auftrag von tran at theochem.tuwien.ac.at<mailto:tran at theochem.tuwien.ac.at> [tran at theochem.tuwien.ac.at<mailto:tran at theochem.tuwien.ac.at>]
Gesendet: Montag, 24. Juli 2017 14:47
An: A Mailing list for WIEN2k users
Betreff: Re: [Wien] experimental vs relaxed parameters using wien2k

Hi,

If you are not interested in structural properties because
this is not the topic of the paper you want to write, then just use
the available experimental structure for band structure, optics, etc.

When no experimental structure is available, then you have no choice
but the problem is to choose the appropriate functional to calculate
the structure.

FT

On Monday 2017-07-24 14:27, Chouaib AHMANI FERDI wrote:

>Date: Mon, 24 Jul 2017 14:27:43
>From: Chouaib AHMANI FERDI <ahmaniferdichouaib at gmail.com<mailto:ahmaniferdichouaib at gmail.com>>
>Reply-To: A Mailing list for WIEN2k users <wien at zeus.theochem.tuwien.ac.at<mailto:wien at zeus.theochem.tuwien.ac.at>>
>To: wien at zeus.theochem.tuwien.ac.at<mailto:wien at zeus.theochem.tuwien.ac.at>
>Subject: [Wien] experimental vs relaxed parameters using wien2k
>
>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
>
>
>[signature.gif?u=139409&e=8673185&v=632ab2fe5f7587109472ad46fdedd2b235e7342
>ab64c99b4930f7b42d8fd3fb2]
>
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--
AHMANI FERDI Chouaïb
Ph.D Student in Material Science
Ecole Normale Supérieure
Mohammed V University, Rabat.
Tel : +212 6 94 59 57 60