[Pw_forum] different PP gives different result, why???

[Giovanni Cantele]

On 05 Feb 2014, at 10:56, Ganjar Kurniawan <zargan88 at yahoo.com> wrote:

> Dear QE
> 
> I am trying to compare the calculation of the Lithium bulk system which has BCC structure with lattice constant is 6.597 bohr using different pseudopotential between Li.pbe-s-rrkjus_psl.0.2.1.UPF and Li.pw91-n-van.UPF.
> 
> the result give different of total energy are -14.82521305 Ry and -1.82426227 Ry respectively...
> 
> The question is why this is happen??? anyone can to explain it???
> 
> the response very appreciated
> 
> Best regard
> 
> Ganjar Kurniawan S
> 
> <lidoang.in><lidoang_vcrelax.out><lidoang_vcrelax2.out>_______________________________________________
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Usually, you should worry in the case two different pseudo potentials gave you the same total energy.
Indeed, the total energy is totally meaningless in  a plane-wave calculation, the only physically meaningful
quantities being total energy differences.

This being said, in the specific case of yours Li pseudo potentials, it seems (after carefully looking inside to either
your *.out files or to the pseudo potential files), while one of the two pseudos just contains the 2s electron in the valence
(Zval=1.0), the other also contains also the two 1s electrons (Zval=3.0). So, in one case also the “core” electron contribution
to the total energy is included.

Moreover, just in the Li.pbe-s-rrkjus_psl.0.2.1.UPF file, you can read:
    Suggested minimum cutoff for wavefunctions:  78. Ry
    Suggested minimum cutoff for charge density: 355. Ry

Such a high value of the suggested cutoff (despite the pseudo is US), is likely toy be due just to the correct treatment
of the two (much more localised) core electrons).

Hope this helps,

    Giovanni

-- 

Giovanni Cantele, PhD
CNR-SPIN
c/o Dipartimento di Fisica
Universita' di Napoli "Federico II"
Complesso Universitario M. S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
e-mail: giovanni.cantele at spin.cnr.it
Phone: +39 081 676910
Skype contact: giocan74

ResearcherID: http://www.researcherid.com/rid/A-1951-2009
Web page: http://people.na.infn.it/~cantele

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