[Pw_forum] Negative charge

[Paolo Giannozzi]

Usually negative charges tend to disappear, not to increase,
at large cutoff, but it is hard to say more.

P.

On Nov 8, 2012, at 15:37 , Latévi Max LAWSON DAKU wrote:

> Dear QE users and developers,
>
>
> I am working on a large system, which I would like to characterize
> in two spin states. The unit cell is made of 324 atoms and there are
> 1220 electrons. The system is made of transition metal complexes
> in a polymeric framework; and I am using the PBE GGA and USSPs
> with nonlinear core corrections, a few of them which I have generated
> and tested as much as possible.
>
> To proceed, I have taken in the USPPs the suggested largest cutoffs,
> which I have raised to WFNc=70Ry and RHOc=750Ry, for the kinetic
> energy and charge density, respectively.
> I ran optimization calculations (vc-relax)  with the total  
> magnetization
> fixed to the desired values and the Gamma point only. The results
> obtained for the structures in both spin states seem fine and  
> consistent
> with available experimental data for similar and isostructural  
> systems.
> To be sure about the choice made for the k-point sampling, I have
> restarted the optimizations using a shifted 4x4x4 MP grid (actually
> only 4 k-points): this did not change the results; this seems fine.  
> So I
> felt quite confident.
>
> As a further test, and before moving to more demanding phonon
> calculations, I increased the WFNc to 80Ry and RHOc to 950 Ry:
> the cell parameters slightly increases by less than 1%, and the energy
> difference between the two phases changes by less than
> 2% (total energies - quite large values - vary by less than 0.01%).
> However, for these cutoff values, a negative charge (rho_dwn)
> appears in one case, with a final value of the order of 1e-4:
>
>       Total force =     0.006379     Total SCF correction =      
> 0.000024
>
>       entering subroutine stress ...
>
>       negative rho (up, down):  0.000E+00 0.302E-03
>
> which, if I correctly understand the FAQ entry 6.12, is not harmful  
> at all.
> I am however worried by the fact that this charge was not produced
> in the calculations with the lower cutoff values. Did one of you  
> already
> observe this? Should this be taken as a symptom of something wrong
> in the calculation setups (USPPs, or WFNc and RHOc values)?
>
> Thanks for your time, answers and comments.
>
> Best regards,
> Max
>
>
> -- 
> Latevi Max LAWSON DAKU
> Universite de Geneve - Sciences II
> Quai Ernest-Ansermet 30                            Tel: (41) 22/379  
> 6548
> CH-1211 Geneve 4                                   Fax: (41) 22/379  
> 6103
>
>
>
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---
Paolo Giannozzi, Dept of Chemistry&Physics&Environment,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222