[Pw_forum] R: Fermi energy

[DELLACA' Valentina (CRF)]

Dear Giuseppe,
I've read the papers you suggested, and indeed they helped a lot. Now I have a further question regarding the Delta V: since we are working with high defect concentrations, the cell parameters change going from the non defective bulk to the defective cells. With VESTA I can plot  the  potential in a 2D plane obtained by slicing  the cell. How can I evaluate the potential at the same point in the defective and non defective cell, considering that the cell parameters are different? Is there a trick I can use?
Thank you very much for your help,
Valentina

Valentina Dellacà
Group Materials Labs
PA&CT – Materials CAE Development & Virtual Analysis 

 
Centro Ricerche Fiat S.C.p.A.
Sede legale e amministrativa: Strada Torino, 50
10043 Orbassano (TO), Italia
Tel  +39 011 9083138
Fax +39 011 9083666

www.crf.it 

-----Messaggio originale-----
Da: pw_forum-bounces at pwscf.org [mailto:pw_forum-bounces at pwscf.org] Per conto di Giuseppe Mattioli
Inviato: mercoledì 28 maggio 2014 12:07
A: pw_forum at pwscf.org
Oggetto: Re: [Pw_forum] Fermi energy


Dear Valentina

> Is it already the Fermi Energy defined in the article by Zunger?

Definitely not! You should individuate the valence band maximum and use the corresponding eigenvalue in the formula as a constant. The Fermi level is the variable (you calculate the formation energy at a given Fermi level, and, possibly, the transition level between two different charge states of the same defect). The rationale is that the position of transition levels should be independent on the Fermi level. I may suggest (but do not tell prof. Zunger, please...) that Van de Walle's paper is somewhat clearer:

Van de Walle, C. G.; Neugebauer, J.; J. Appl. Phys. 2004, 95, 3851.

A very recent review by Pacchioni and Di Valentin, which improves in some details the calculation of transition levels, can be found here:

http://pubs.acs.org/doi/pdf/10.1021/ar4002944

There is also another detail, often hidden into the papers. When you perform the calculations in a plane-wave/pseudopotential PBC framework, if you change the total charge, you also insert a background charge that shift the potential (defined up to a constant
value) and, in turn, the eigenvalues. You should insert some DeltaV value in the formation energy formula to recover the shift. If the supercell is large, and the dielectric constant of the host matrix is high (let us say a >100-atom cell of TiO2, for instance), then DeltaV is negligibly small. Otherwise you can find something around some tenths of eV. However, Pacchioni and Di Valentin have tested a workaround for the estimate of DeltaV (see the review).

> occupations='smearing' with degauss =1d-20

I hope that it is degauss =1d-2

HTH

Giuseppe

Giuseppe Mattioli
ISM-CNR
Italy

Quoting "DELLACA' Valentina (CRF)" <valentina.dellaca at crf.it>:

> Hi,
> We are computing the formation energies of charged defects in 
> supercells. We are referring to Lany and Zunger, Phys. Rev. B 78,
> 235104 (2008) Eq. (1).
> The Fermi energy used here is defined as  Ev (valence band maximun)
> + delta E Fermi ; i.e. is defined with respect the valence band
> maximum.
> The system we are working with is an insulator, and we are using 
> occupations='smearing' with degauss =1d-20 in order to have the Fermi 
> Energy written in the log file.( Please, tell me if we are doing 
> something wrong here!) The question is: the Fermi Energy that I read 
> in the log file, is defined respect to what? Is it already the Fermi 
> Energy defined in the article by Zunger?
> Thanks,
> Regards
> Valentina
>
>
> Valentina Dellacà
> Group Materials Labs
> PA&CT - Materials CAE Development & Virtual Analysis
>
>
> Centro Ricerche Fiat S.C.p.A.
> Sede legale e amministrativa: Strada Torino, 50
> 10043 Orbassano (TO), Italia
> Tel  +39 011 9083138
> Fax +39 011 9083666
>
> www.crf.it<http://www.crf.it>


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