[Pw_forum] GGA underestimates lattice constant in BaO

[Munima Bora Sahariah]

Dear Dr. Toffoli

thanks for putting your comments and interest in this matter.

The pbe pseudopotential that I took from pwscf already has 5S, 5P, 5D, 6S
and 6P orbitals included. I am not sure whether inclusion of more orbitals
(4P at least) would make any difference. We can give it a try.

yesterday I tried with Ba and O  GGA pseudopotentials taken from ABINIT.
they are norm conserving ones. I found the lattice constant for BaO too
much overestimated.

I will let you know if I find some more information.

regards

munima

> Dear Munima,
>
> I also ran into the exact same problem just yesterday using the PBE
> pseudopotential from the web site. The lattice constant that you quote is
> almost identical to the one I obtained. Inspired by your post, I
> calculated
> the lattice constant for BaS to compare to the result in the paper you
> quote and it matches better (almost the same as the experimental value). I
> don't know if this is supposed to make us feel better.
>
> I also thought of generating a new pseudopotential but as far as I know,
> the generator cannot include two orbitals of the same l even though they
> might be from different shells. Thus we couldn't include 5s and 6s
> orbitals
> simultaneously. Am I right?

>I would also appreciate thoughts on what we
> could do or pseudopotentials that have been converted from other codes if
> there are any lying around.
>
> Thank you very much.
>
> Best,
> Hande
>
> On Fri, 1 Aug 2008, Paolo Giannozzi wrote:
>
>>
>> On Jul 23, 2008, at 7:58 , Munima Bora Sahariah wrote:
>>
>> > For last couple of days I have been trying to find the equilibrium
>> > lattice
>> > constant of BaO with GGA (pw91) and has found that  my calculated
>> > value
>> > underestimates the experimental one by 1.3% (GGA = 10.301 a.u., EXP =
>> > 10.431 a.u.). Surprizingly, while trying with LDA, I obtained a value
>> > which is closer to the experimental one as compared to GGA (LDA =
>> > 10.324
>> > a.u.) [...]
>> > While doing a search on the web, I found one paper (Phys. Rev. B 71,
>> > 085203) where GGA and LDA results for lattice constant of BaO have
>> > been
>> > listed. In this paper calculations were done in CASTEP. Here the
>> > LDA value
>> > underestimates the experimental lattice constant while the GGA
>> > (pw91) value
>> > improves the situation and is closer to the experimental one.
>>
>> try different pseudopotentials if you can: this might be one of those
>> cases
>> in which the results are more sensitive than desired to the quality of
>> pseudopotentials  (in particular of Ba).
>>
>> Paolo
>> ---
>> Paolo Giannozzi, Dept of Physics, University of Udine
>> via delle Scienze 208, 33100 Udine, Italy
>> Phone +39-0432-558216, fax +39-0432-558222
>>
>>
>>
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>
> --
> Hande Toffoli
> Department of Physics
> Office 439
> Middle East Technical University
> Ankara 06531, Turkey
> Tel : +90 312 210 3264
> http://www.physics.metu.edu.tr/~hande
>
>
>
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