Thanks Giuseppe!
Indeed, with Oliviero we studied this class of problems (unbound anions,
and their absorption on metal slabs). The discussion is here:
https://pubs.acs.org/doi/abs/10.1021/acs.jctc.9b00552 (also on arxiv).
In a nutshell, the "correct" bound state is there, but in vacuum the
unbound state is lower in energy, so that's where DFT-PBE converges to.
In the presence of continuum solvation, the "correct" bound state
becomes the lowest in energy, so that's where the calculation converges
to. Since these energies are linear in the dielectric constant of the
medium, you can extrapolate to the vacuum case.
I say "correct" because the anion would still have self-interaction
errors - they seem to be smaller than I would have thought, meaning the
the damaging effect of self-interaction is stabilizing the wrong state,
but the correct "excited" state has good energies. Same reason why
penalty functionals or constrained-DFT work well for charge-transfer
excitation - they do not fix the errors of approximate DFT, but allow
you to select your "ground" state that approximate DFT makes an excited
state. Piece of cake...
nicola
On 19/05/2020 09:45, Giuseppe Mattioli wrote:
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 <stan...@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
Via Salaria Km 29,300 - C.P. 10
I-00015 - Monterotondo Scalo (RM)
Mob (*preferred*) +39 373 7305625
Tel + 39 06 90672342 - Fax +39 06 90672316
E-mail: <giuseppe.matti...@ism.cnr.it>
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Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
Director, National Centre for Competence in Research NCCR MARVEL, EPFL
http://theossrv1.epfl.ch/Main/Contact http://nccr-marvel.ch/en/project
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