[QE-users] large negative rho

[K. Doblhoff-Dier]

Dear Nicola,

Thank you for your answer and sorry for my late reply. The argument with 
the Gibbs phenomena would make sense to me. Your argument, however, that 
this should have become obvious in the pseudopotential generation does 
not become clear to me yet: Would the Vanderbilt code not only complain, 
if it were to suggest 'real' negative densities? ('real' meaning "before 
interpolating the augmentation charges to a grid and then evaluating it 
on an even finer grid").
I guess the question is not the most pressing one - for the time being I 
simply switched to a different pseudopotential. It is more my own 
curiosity that drives me to wanting to understand the issue. Also, it 
seems to me as if the presence of negative densities weren't really 
hurting the calculations (except for the fact that they are somewhat 
tricky to converge), suggesting that one may be able to ignore the 
warning, but I am not willing to do so before I haven't understood its 
origin and can judge on a more scientific basis than "from the few 
calculations I did it seems as if...".

Thank you and best regards,

Katharina


On 04/01/2019 21:20, Nicola Marzari wrote:
>
>
> Dear Katharina,
>
>
> good reminder, and I'm not sure what the consensus is.
>
> I vaguely recall that if the augmentation charges are "hard"
> (very localized), the pseudization of the q_ij function
> ifqopt,nqf,qtryc,nfix (see discussion in
> http://physics.rutgers.edu/~dhv/uspp/uspp-736/Doc/INPUT_GEN)
> can give rise to a Gibbs phenomenon (if you try to interpolate
> something hard like a step, you go negative).
>
> For this reason, the CP code (that resricts to the small boxes nr1b
> nr2b nr3b with fine resolution the action of the uspp) doesn't suffer 
> from this problem, that happens away from the core (PWscf has fine 
> resolution everywhere, in the action of the uspp).
>
> Now, this should also have been seen in the pseudopotential generation
> (see "negative densities" in 
> http://physics.rutgers.edu/~dhv/uspp/uspp-736/Doc/TUTORIAL), so I'm 
> the first one at a loss (of density).
>
>                 nicola
>
>
>
>> Dear QE users and developers,
>>
>> I performed DFT calculations using the grvb-1.4 uspp for Pt (just a 
>> single atom in a cell) and I get rather large negative values for rho 
>> even after scf convergence:
>>
>> -----snippet of the pwscf log 
>> file-----------------------------------------------------
>>
>>       negative rho (up, down):  1.759E-01 0.000E+00
>>       total cpu time spent up to now is       41.4 secs
>>       End of self-consistent calculation
>>       Number of k-points >= 100: set verbosity='high' to print the 
>> bands.
>>       the Fermi energy is    22.2850 ev
>> !    total energy              =    -210.69743345 Ry
>>
>> ------end of 
>> snippet--------------------------------------------------------------------- 
>>
>>
>> Usually, I would have thought that this is a sign of a too low 
>> ecutwfc (or actually ecutrho). But, while very similar results were 
>> obtained for ecutwfc=35Ry and ecutrho=280Ry, the above results were 
>> obtained for ecutwfc=100Ry and ecutrho=1000Ry, which should be 
>> crazily large for the PP in question.
>>
>> I guess, one might say that -0.17e can be safely ignored (see the 
>> pwscf faq), and the lattice constant and bulk modulus are indeed 
>> totally reasonable. However, the error scales with number of atoms, 
>> so once I go to a Pt slab, I get negative charges around -3e and I am 
>> hesitant to accept that without thinking twice.
>>
>> A very similar question has been posed before 
>> (https://www.mail-archive.com/users@lists.quantum-espresso.org/msg18381.html), 
>> but has not been answered.
>>
>> Any hints?
>>
>> Thanks in advance
>>
>> Katharina Doblhoff-Dier
>>
>>
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>