[Q-e-developers] Problems converging anions in vacuum

[Lorenzo Paulatto]

On Fri, 11 May 2012 12:04:30 +0200, Oliviero Andreussi  
<oliviero.andreussi at epfl.ch> wrote:
> Just to make things more reproducible, I rerun some tests with clean
> versions of QE, both the svn and the 4.3.2, the latter also using the
> gamma only algorithm, you can find the input and outputs attached. There
> are some small differences in the numbers, but the overall effect is the
> same, the SCF goes nuts. I would like to stress that the problem is not
> system specific, the same happens for the input atomic positions
> translated or in larger cells, with or without martyna-tuckerman
> correction, and the same happens for many anions (10 out of 60,
> including OH-),

Dear Oliviero,
I'm not 100% sure that what I-m about to say is right, but I think it is  
the explanation of your problems. I think that one electronic state may be  
very little bound, or not bound at all, in DFT hence the electron prefers  
to delocalize instead on staying on the molecule. When the delocalization  
happens, it is very hard to make the system converge, and even if it does  
the result will depend strongly on the cell size and other factors that  
should not really matter.

Anyway, this would not be a bug in the code, but a limit of the functional  
that is causing a pathological case. I guess Martina-Tuckerman would not  
help as it does not act on the bands, on the other hand vdw-DF or EXX  
could help. Less radical "solutions" may include: increase the number of  
bands, add a small smearing and, most effectively, reduce the size of the  
unit cell. I reckon that the latter could also introduce spurious  
interaction between periodic images, but there are corrections for that.

bests


-- 
Lorenzo Paulatto IdR @ IMPMC/CNRS & Université Paris 6
phone: +33 (0)1 44275 084 / skype: paulatz
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