[Wien] volume optimisation of Fe2O3

[Chandrika]

Dear Torsten and wien users,
I tried with other options for the Fe2O3 cell (LDA-5, GGA-14 and GGA-13  ).
The best is with GGA (13) which optimises around -12%. I also did mini
followed by volume optimisation and c/a ratio optimisation. After mini xyz
positions change somewhat. Ultimately the derived a = 4.825 Aº  and
c = 13.148 Aº. Experimental values are a= 5.035 and c= 13.72.

Could anyone who has derived values closer to the experimental, send their
structure file.

Thank you for your help.

Chandrika



----- Original Message ----- 
From: "Torsten Andersen" <thor at physik.uni-kl.de>
To: "A Mailing list for WIEN2k users" <wien at zeus.theochem.tuwien.ac.at>
Sent: Wednesday, September 13, 2006 12:06 PM
Subject: Re: [Wien] volume optimisation of Fe2O3


Dear Chandrika,

you should be aware that the "optimized" volume depends strongly on the
PARAMETER called the XC-Potential, since there is no universal XC-Pot
that works equally well for everything. DFT usually defines some
accurate terms that can be determined well, and the rest is put into the
XC-Pot (and with "exact" exchange, this is hence called the C-Pot).
Therefore, an XC-Pot that is the best for one material might not be the
best option for another material, and you get large deviations from
experimental data. What is good for Fe is not necessarily good for its
oxides...

If you repeat the optimization with a different XC-Pot you might get
better results.

Best regards,
Torsten Andersen.

Chandrika wrote:
> Thanks Gerhard and Torsten for the suggestions.
> I reduced the convergence criteria and could obtain the optimised value
> at -12.5% volume. Somehow the structure files were getting the x-y-z
> positions all wrong in the various volume files, when I used the more
> stringent criteria.
> On the basis of experimental values I was expecting the volume
optimisation
> to show a minimum around say 5%.
> I studied total energies at different k points and found 600 as the
optimum
> (as suggested in Optimisation Notes).
> Thank you for your help,
> Chandrika
>
> ----- Original Message ----- 
> From: "Gerhard H Fecher" <fecher at uni-mainz.de>
> To: "A Mailing list for WIEN2k users" <wien at zeus.theochem.tuwien.ac.at>
> Sent: Monday, September 11, 2006 7:45 PM
> Subject: Re: [Wien] volume optimisation of Fe2O3
>
>
> I remember that I needed already more than 20 cycles for convergence
> at the experimental lattice parameter with more relaxed settings than you
> do,
> maybe you should increase it , to be shure that the calculations are
really
> converged.
> What is the meaning of   k-points= 600 (optimized) ?
>
> Ciao
> Gerhard
>
>
>>> k-points= 600 (optimized)
>>>
>>>*Without Spin – polarization:*
>>>
>>> run_lapw –I –i 20 –ec 0.00001 –cc 0.0001
>>>
>>>
>>>
>>>*Non spin-polarized but with spin-orbit interaction*:
>>>
>>>run_lapw –so –cc 0.0001.
>>>
>>>Thank you for your suggestions,
>>>
>>>Chandrika
>>>
>>>
>
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-- 
Dr. Torsten Andersen        TA-web: http://deep.at/myspace/
AG Hübner, Department of Physics, Kaiserslautern University
http://cmt.physik.uni-kl.de    http://www.physik.uni-kl.de/
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