[Pw_forum] GGA+U

[Giuseppe Mattioli]

Dear F. Marsusi

> Is this what one expected always from +U calculations,

As I told you before nothing can be considered as "expected" when you apply the U correction to a strongly hybridized sp2 system such as a pi-
conjugated molecule or graphene. In my experience this is not a good idea, because you obtain, for example, a higher ionization energy (and this is 
right when you correct LDA/GGA delocalization errors, and in agreement with a correction with exact exchange like B3LYP or HSE) but also a lower 
electron affinity (and this is wrong, and the opposite behavior of a correction with exact exchange). So you are walking in "terra incognita" where 
nothing can be given for granted.
HTH
Giuseppe

On Friday, February 10, 2017 08:38:37 PM FARAH MARSUSI wrote:
> Dear Giuseppe,
> 
> Many thanks for your quick response. As you have correctly guessed, the system is an organic material (fluorinated graphene). From U=0 to 3.4, M is
> increasing gradually up to the expected value, M remains constant till U=3.6 eV, then by increasing U, again M is reduced very soon to zero. Is
> this what one expected always from +U calculations, when goes up further from the optimum U value? From your answer, it is concluded to be a
> strange behavior due to the organic nature of this material.
> 
> Best regards,
> F. Marsusi,
> 
> Amirkabir University of Technology.​
> 
> On Fri, 02/10/2017 08:09 PM, Giuseppe Mattioli <giuseppe.mattioli at ism.cnr.it> wrote:
> > Dear F. Marsusi
> > First of all you are not providing any detail of your system, so we cannot even guess what is "natural" for it.
> > However, you are using the Hubbard U correction in a semiempirical way, and there is therefore no way to choose the U value but that reproducing
> > some measured parameter, and not very much to do if unexpected or strange results appear at a given (unphysical?, problematic?) U value.
> > In my experience the application of U to organic molecules has been always problematic due to the strong hybridization between C 2s and C 2p
> > orbitals, and to the application of the correction to 2p orbitals only.
> > HTH
> > Giuseppe
> > 
> > On Friday, February 10, 2017 07:25:10 PM FARAH MARSUSI wrote:
> > > Dear all,
> > > 
> > > By GGA+U as implemented in QE, the correct magnetization (M) and band gap was obtained. The correct U value for each atom was obtained by
> > > intensive
> > > step by step runs to reach gradually the experimental M value, and therefore band gap. All results are OK till now (the U value itself also= 3.4
> > > eV
> > > for carbon and fluorine is acceptable ). However, by enlarging the obtained U value a bit (to 3.7 eV), the predicted M come back to that of pure
> > > GGA. Is it natural, or a problem exist?
> > > 
> > > 
> > > Best regards,
> > > F. Marsusi,
> > 
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> >    Giuseppe Mattioli
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********************************************************
- Article premier - Les hommes naissent et demeurent
libres et égaux en droits. Les distinctions sociales
ne peuvent être fondées que sur l'utilité commune
- Article 2 - Le but de toute association politique
est la conservation des droits naturels et 
imprescriptibles de l'homme. Ces droits sont la liberté,
la propriété, la sûreté et la résistance à l'oppression.
********************************************************

   Giuseppe Mattioli                            
   CNR - ISTITUTO DI STRUTTURA DELLA MATERIA   
   v. Salaria Km 29,300 - C.P. 10                
   I 00015 - Monterotondo Stazione (RM), Italy    
   Tel + 39 06 90672342 - Fax +39 06 90672316    
   E-mail: <giuseppe.mattioli at ism.cnr.it>
   http://www.ism.cnr.it/en/staff/giuseppe-mattioli/
   ResearcherID: F-6308-2012