[Wien] Convergence problem in mbj potential calculation

pieper pieper at ifp.tuwien.ac.at
Thu Jun 15 12:52:35 CEST 2017 

You keep getting back to this match between the (presumably powder-)xrd 
you calculated from your .struct file and the experimental one. This 
does NOT imply that your structural model describes the physical 
situation correctly! With your - in Gerhards words - 'interesting 
approach' you should be able to produce any number of different .struct 
files, and calculating their powder-xrd find a similar 'match with the 
experimental one'.

Did you calculate the xrd with any of the alternative distributions of 
Li, Ni and Ti on their sites in your unit cell? Did you try the same 
with the many, many other distributions you can generate in larger 
supercells? I am prettty sure you will find something that matches your 
xrd data. It will make as much sense as what you have now ...

Be aware of the fact that structural models of Wien2k have symmetry 
properties  different from your physical situation: in your model the 
metal atoms occupy lattice sites regularly, breaking certain symmetries 
of the underlying lattice. The actual distribution in your case is 
(apparently more or less) random, which overall breaks no symmetry, but 
locally breaks every symmetry somewhere.

If the distribution of Li, Ni, and Ti  really is random on one shared 
site I know of only one (expensive) way to proceed: try to reduce the 
influence of short range periodicity in your models by increasing the 
size of the supercell and putting Li, Ni, and Ti at random positions. Do 
structural relaxations, look what changes in your (GGA-) calculations, 
and keep fingers crossed that things you are interested in converge for 
supercells of a size you still can handle.



---
Dr. Martin Pieper
Karl-Franzens University
Institute of Physics
Universitätsplatz 5
A-8010 Graz
Austria
Tel.: +43-(0)316-380-8564


Am 15.06.2017 07:12, schrieb shamik chakrabarti:
> Dear Gerhard,
> 
>                 You are right. The system can be written as
> (Li0.5Ti0.25Ni0.25)O.
> 
> However, my mbj simulation is converged with MSR1 mixing in 221
> cycles. And the DOS has come as metallic in this case too. Now, I may
> try -eece also. So far, in GGA, GGA+U and in mbj the DOS has come as
> metallic. However, if the structure is wrong it will not give the
> correct DOS in any case. Although, the xrd is matched with
> experimental one, there is an issue of longer Ti-O bond. I have to see
> how can I get the correct structure.
> 
> Any suggestion in this regard will be helpful for us.
> 
> with regards,
> 
> with regards,
> 
> On Wed, Jun 14, 2017 at 8:00 PM, Fecher, Gerhard <fecher at uni-mainz.de>
> wrote:
> 
>> Interesting approach
>> as in Li2NiTiO4 the Li, Ni, and Ti atoms are randomly distributed on
>> the 4a site with oxygen on 4b of the NaCl lattice
>> therefore one better should write (Li0.5Ti0.25Ni0.25)O
>> 
>> 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
>> shamik chakrabarti [shamikphy at gmail.com]
>> Gesendet: Mittwoch, 14. Juni 2017 15:39
>> An: A Mailing list for WIEN2k users
>> Betreff: Re: [Wien] Convergence problem in mbj potential calculation
>> 
>> Dear Gerhard & Laurence,
>> 
>> Thank you very much for your
>> reply & suggestions. The structure was initially of Li2NiTiO4. The
>> space group is Fm-3m. We have created a 1x1x1 supercell (with target
>> lattice primitive) and then distributed Li, Ni & Ti with 2:1:1
>> ration at 4b site. 4a site contains oxygen. Then we have optimize
>> the volume and force per atom of the structure sequentially &
>> results in the present structure. The xrd generated by the present
>> structure matches exactly with the experimental xrd of Li2NiTiO4.
>> 
>> However, we have shifted the mixing procedure from PORT to MSR1 and
>> the simulation is converging well now.
>> 
>> with regards,
>> 
>> On Wed, Jun 14, 2017 at 3:17 PM, Laurence Marks
>> <L-marks at northwestern.edu<mailto:L-marks at northwestern.edu>> wrote:
>> I have said this before, but it is worth repeating: the most common
>> reason a calculation does not converge is because the
>> structure/model is unphysical/unchemical. Your Ti-O bonds are too
>> long, the structure is "obviously" wrong.
>> 
>> Scientific rigor is not optional. What is the R-factor? Are the
>> temperature factors sane? Is there an independent​ measurement of
>> the composition, e.g. ICP? Have you measured that it is FM?
>> 
>> ---
>> Professor Laurence Marks
>> "Research is to see what everybody else has seen, and to think what
>> nobody else has thought", Albert Szent-Gyorgi
>> http://www.numis.northwestern.edu [1]
>> Corrosion in 4D http://MURI4D.numis.northwestern.edu [2]
>> Partner of the CFW 100% gender equity project,
>> www.cfw.org/100-percent [3]<http://www.cfw.org/100-percent [3]>
>> Co-Editor, Acta Cryst A
>> 
>> On Jun 14, 2017 04:24, "Fecher, Gerhard"
>> <fecher at uni-mainz.de<mailto:fecher at uni-mainz.de>> wrote:
>> Indeed, your compound is also metallic already with PBE, this you
>> did not tell when I asked about PBE.
>> Also, the PBE does not result in a ferromagnetic state, I guess ?
>> 
>> --
>> 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 [1]<http://www.numis.northwestern.edu
>> [1]><http://www.numis.northwestern.edu [1]> ; Corrosion in 4D:
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>> Partner of the CFW 100% program for gender equity,
>> www.cfw.org/100-percent [3]<http://www.cfw.org/100-percent [3]>
>> 
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>> --
>> Dr. Shamik Chakrabarti
>> Post Doctoral Research Associate
>> Dept. of Condensed Matter Physics and  Material Science
>> S N Bose National Centre for Basic Sciences
>> Kolkata-700098
>> INDIA
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>> --
>> Dr. Shamik Chakrabarti
>> Post Doctoral Research Associate
>> Dept. of Condensed Matter Physics and  Material Science
>> S N Bose National Centre for Basic Sciences
>> Kolkata-700098
>> INDIA
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>> Wien mailing list
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> 
> --
> 
> Dr. Shamik Chakrabarti
> Post Doctoral Research Associate
> Dept. of Condensed Matter Physics and  Material Science
> S N Bose National Centre for Basic Sciences
> Kolkata-700098
> INDIA
> 
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