[QE-users] Convergence problem with hydroxide

[Giuseppe Mattioli]

Dear Jeremy
It is not a technical problem, it is a theoretical one.
negative ions in gas phase generally yield unbound excess electrons  
leading to weird results when computed at the GGA level of theory. Of  
course water stabilizes OH-, and also exact exchange if you use a  
hybrid functional.
HTH
Giuseppe

Quoting RABONE Jeremy <jeremy.rabone at get.omp.eu>:

> Hello,
>
>   I have just started a project using QE, and have a question  
> regarding a hydroxide ion in a box. When I first tried to do a  
> relaxation with conv_thr = 1.0d-8, it started off like it was going  
> to converge and then suddenly the convergence went all over the  
> place. Changing all of the other parameters made no difference, but  
> reducing
> conv_thr to 1.0d-6 stops the scf step before the problem and the  
> calculation completes. There also seems to be some effect of the  
> randomised starting wavefunctions even with conv_thr = 1.0d-6  
> (sometimes converges, sometimes goes all triplets). When surrounding  
> the ion with a water cage, the calculation works with no problems.  
> If I switch to a spin polarized calculation, with conv_thr = 1.0d-8  
> it seems determined to converge to a triplet state with the first  
> antibonding orbital occupied, and does so with no difficulty is the  
> magnetization is set to 2.
>
> I recall there were sometimes issues in DFT when the order of state  
> energies changes during a calculation - is this what is happening  
> here and is there a way to get passed the problem or do I just have  
> to be content with a reduced threshold ?
>
> A S2- ion in a box does something similar - seemingly converging to  
> triplet with a d-state occupied, and even when surrounded by a water  
> cage it fails to converge in a singlet state (although in this case  
> one might question the validity of the calculation).
>
> I'm using SG15 norm-conserving pseudopotentials but the PAW  
> pseudopotential seems to do the same thing.
>
> Regards,
>
> Jeremy



GIUSEPPE MATTIOLI
CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
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