[Pw_forum] GGA underestimates lattice constant in BaO

[Hande Ustunel]

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
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