[QE-users] Charged Convergence Issues

[Giuseppe Mattioli]

Dear Robert
The calculation does not converge due to delocalization error (see,  
e.g., Cohen et al., Science 2008, 321, 792), which affect local (LDA)  
and semilocal (GGA) functionals. The excess electron of COO- is  
unbound in your DFT description of the system and hinders scf  
convergence. On the other hand, COO- is not stable in gas phase, while  
it is stable in water solution. There are a few possible things to do:

1) use a hybrid GGA+EXX functional that partially corrects the  
delocalization error. It helps but COO- (gas) may still contain an  
unbound electron.
2) embed your system in an implicit solvent with the QE plugin (see  
quantumenviron.org for details)
3) use 2) together with the explicit inclusion of a first solvation  
sphere (e.g., a few explicit water molecules) which lower the energy  
of the unbound electron and stabilize COO-, favoring convergence.
HTH
Giuseppe

Quoting Robert Stanton <stantor at clarkson.edu>:

> Hello,
>
>     I'm having an issue in calculating adsorption energies in a system with
> charged molecules. I have relaxed the structure with and without the
> adsorbent. In the case of structure+adsorbent, I introduce a compensating
> jellium background charge for the COO- molecule, and this converges fine.
> The structure itself is neutral and converges fine in a neutral cell.
> However I need to also optimize the lone COO-, and I am running into the
> issue that this will not converge in the case of a charged cell, but only a
> neutral one. The issue then is that if I use the COO- energy converged in
> the neutral cell, I get adsorption energies that are not accurate.
>
>     I am mainly just wondering how the calculation is converging in the
> latter two cases above, since from what I've read it seems one of them
> should have a charged cell causing convergence issues? Could cutoffs
> potentially be causing an issue here? I have tried adding spin
> polarization, dropping the mixing beta extremely low, and tried other small
> molecules with a formal charge and they all have a similar issue. Just
> looking for any ideas as to how I could get this charged molecule to
> converge alone in a charged cell.
>
> Thank you in advance,
> Robert Stanton
> Clarkson University



GIUSEPPE MATTIOLI
CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
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E-mail: <giuseppe.mattioli at ism.cnr.it>