[Pw_forum] Negative charge

[Latévi Max LAWSON DAKU]

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