[Pw_forum] Hubbard for oxygen

[Giuseppe Mattioli]

Dear Youssuf
Sorry, I do not fully understand your question, so I try to answer but it might not be what you expect.
in my experience it is better to use a supercell to avoid spurious interactions between periodically replicated images of the U-perturbed atom. The 
response to the alpha perturbation is generally linear around the converged value (U_in=U_out), but when U_in is far from the converged value you may 
found a weird behavior. If you have a good U_out value coming from a unit-cell calculation and you are looking for a converged value with respect to 
the cell dimensions, then use such U_out value as U_in value in a supercell calculation (with k-points scaled to sample in the same, or very close way 
the Brillouin zone). You should see a shift of the new U-out value, depending on the different interaction between perturbed atoms in PBC. Try again to 
start with such U_out as U_in until you reach convergence (0.1 eV is generally enough for standard purposes). Then you may try again to extend the 
supercell and to check if your last U_out value is stable in the new system...
HTH

Giuseppe 

On Sunday, February 15, 2015 12:23:37 PM Youssef wrote:
> Dear Giuseppe,
> 
> Thank you very much for your help, I was really stuck.
> 
> I have another question about the linear response approach, in fact I
> successfully got the liner behavior of alpha=f(dn) and alpha=f(dn0) on
> conventional cell, with a very good value of U with respect to the
> literature, now when I move to supercell (2x2x2) the linearity is
> broken !!!
> 
> Any idea about this issue?
> 
> P.S: The convergence criterion's are well tested in addition to cell and
> atomic positions are relaxed

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   Giuseppe Mattioli                            
   CNR - ISTITUTO DI STRUTTURA DELLA MATERIA   
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