[Wien] experimental vs relaxed parameters using wien2k

Tue Jul 25 13:31:17 CEST 2017

Differences between DFT lattice parameters and experiment are well known,
although smaller (as you have) is less common. (Maybe because you did not
use +U).

For "better" results, there are many methods one common being PBEsol. There
is a large literature on this. Of course, whether this is really better is
less clear -- why should the lattice parameters at T=0K be the same as
experiment, and how clean is the sample in experiment?

On Tue, Jul 25, 2017 at 5:49 AM, Chouaib AHMANI FERDI <
ahmaniferdichouaib at gmail.com> wrote:

> 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,
>
>
>
>
> On Mon, Jul 24, 2017 at 2:09 PM, Fecher, Gerhard <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] im Auftrag von
>> tran at theochem.tuwien.ac.at [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>
>> >Reply-To: A Mailing list for WIEN2k users <wien at zeus.theochem.tuwien.ac.
>> at>
>> >To: 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=632ab2fe5f7587109472ad4
>> 6fdedd2b235e7342
>> >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
>
>
>

-- 
Professor Laurence Marks
"Research is to see what everybody else has seen, and to think what nobody
else has thought", Albert Szent-Gyorgi
www.numis.northwestern.edu ; Corrosion in 4D: MURI4D.numis.northwestern.edu
Partner of the CFW 100% program for gender equity, www.cfw.org/100-percent
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