[QE-users] Charged Convergence Issues

[Robert Stanton]

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
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