[Wien] Supercell calculation is not converging

[Laurence Marks]

Check the confidence levl, e.g. t-test (i am not an expert with this
package). Xrd Reitfeld is as susceptible to GIGO as DFT.
On Sep 5, 2011 11:58 AM, "shamik chakrabarti" <shamikphy at gmail.com> wrote:
> Dear Laurence Marks Sir,
>
> Yes this is an oxide and it is an insulator and probably I have made it
> a metal by creating *cationic vacancies*. Sir I will let you know the
detail
> after checking it from case.output2. But Sir, please forgive my curiosity,
I
> could not wait to ask you and also going a bit away from wien2k questions:
> 1) Sir the goodness of fit (GOF) obtained from Rietveld analysis done
> through XPERT Highscore plus : 1.0038
> and except this data we do not have any other proof to show that it
> really have cationic vacancies.
> Then Sir according to you we can not trust this data (considering GOF)
> only to consider cationic vacancies?
>
> With best regards,
>
>
>
> On Mon, Sep 5, 2011 at 9:31 PM, Laurence Marks <L-marks at northwestern.edu
>wrote:
>
>> Follow the science, not the DFT.
>>
>> 1) Unless you have a large grain size and synchotron data I really
>> wonder about the logic of removing 1 atom in a 2x1x1 supercell.
>> Particularly if this is an oxide, have you made it a metal whereas it
>> should be an insulator? Does the fit pass significance tests etc? Do
>> you have other data that implies vacancies?
>> 2) In terms of the DFT, I expect that you have made it a metal which
>> is harder to converge than an insulator. Check in case.output2 in both
>> cases (a quick, pseudo DOOS). If it really does have vacancies, you
>> might need more than 7 kpts, and I suggest using TEMPS.
>>
>> 2011/9/5 shamik chakrabarti <shamikphy at gmail.com>:
>> > Dear wien2k users,
>> > I have performed a 56 atom / unit cell calculation for a
>> > ferrimagnetic material. It was well converged both in energy and charge
>> by
>> > 60 iterations. 14 K points were used for this calculation. Then I have
>> > generated a 2x1x1 supercell having 112 atoms / supercell. This was done
>> to
>> > achieve better experimental model as obtained through Rietveld
analysis.
>> > From Rietveld analysis it was also seen that for 112 atoms we can
>> consider a
>> > cation (Fe) vacancy and we have generated it by deleting one Fe atom
from
>> > the supercell. Final structure, hence, contains 111 atoms and all other
>> > occupancies at different lattice sites are well matched with Rietveld
>> > analysis of experimental XRD. I am using 2 k points for this 111 atom
>> > supercell. But even after 60 iterations the convergence achieved are as
>> > follows:
>> > cc: 0.234... cc was set to 0.001
>> > ec: 0.063... ec was set to 0.0001
>> > Sir, my question are:
>> > 1) As the supercell is merely an extension of the 56 atoms unit cell
with
>> > more accurate values of atomic occupancies as in the experimental
>> structure,
>> > why it should at all taking so much time to converge?
>> > 2) We have created one defect by creating a Fe vacancy...is it that the
>> > structures having defect take more time to converge?...but as far as
our
>> > analysis until now..the structure should have one Fe vacancy per 112
>> > atoms...
>> > 3) As we have obtained converged solutions for 56 atom cell by using 14
k
>> > points..is it that we should use 7K points for 111 atom supercell
>> > calculation?..is 2 K points are actually small in order to achieve
>> > convergence of this structure?
>> > Any response in this regard will be very helpful for us. Thanks in
>> advance.
>> > with regards,
>> >
>> > --
>> > Shamik Chakrabarti
>> > Senior Research Fellow
>> > Dept. of Physics & Meteorology
>> > Material Processing & Solid State Ionics Lab
>> > IIT Kharagpur
>> > Kharagpur 721302
>> > INDIA
>> >
>> > _______________________________________________
>> > Wien mailing list
>> > Wien at zeus.theochem.tuwien.ac.at
>> > http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
>> >
>> >
>>
>>
>>
>> --
>> Laurence Marks
>> Department of Materials Science and Engineering
>> MSE Rm 2036 Cook Hall
>> 2220 N Campus Drive
>> Northwestern University
>> Evanston, IL 60208, USA
>> Tel: (847) 491-3996 Fax: (847) 491-7820
>> email: L-marks at northwestern dot edu
>> Web: www.numis.northwestern.edu
>> Research is to see what everybody else has seen, and to think what
>> nobody else has thought
>> Albert Szent-Gyorgi
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>>
>
>
>
> --
> Shamik Chakrabarti
> Senior Research Fellow
> Dept. of Physics & Meteorology
> Material Processing & Solid State Ionics Lab
> IIT Kharagpur
> Kharagpur 721302
> INDIA
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