[QE-users] large negative rho

[Nicola Marzari]

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
> 
> 
> _______________________________________________
> users mailing list
> users at lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/users


-- 
----------------------------------------------------------------------
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
Director, National Centre for Competence in Research NCCR MARVEL, EPFL
http://theossrv1.epfl.ch/Main/Contact http://nccr-marvel.ch/en/project