[Pw_forum] GGA underestimates lattice constant in BaO

Hande Ustunel hande at newton.physics.metu.edu.tr
Fri Aug 1 12:29:23 CEST 2008

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.


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
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Middle East Technical University
Ankara 06531, Turkey
Tel : +90 312 210 3264

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