[Wien] Supercell calculation is not converging

[shamik chakrabarti]

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
> _______________________________________________
> Wien mailing list
> Wien at zeus.theochem.tuwien.ac.at
> http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
>



-- 
Shamik Chakrabarti
Senior Research Fellow
Dept. of Physics & Meteorology
Material Processing & Solid State Ionics Lab
IIT Kharagpur
Kharagpur 721302
INDIA
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://zeus.theochem.tuwien.ac.at/pipermail/wien/attachments/20110905/cc07eeec/attachment.htm>