[QE-users] Problem stabilizing a di-anionic molecule

[Nicola Marzari]

Dear Chris,


we discuss this problem quite extensively here: 
https://pubs.acs.org/doi/10.1021/acs.jctc.9b00552

In addition, approximate DFT functionals do a poor job in describing 
these extra electrons, that are very delocalized (interestingly, CCSD(T) 
also struggles because the basis sets don't really extend far enough).

So, not sure there are ideal solutions, and if what you get is actually 
accurate; Perdew-Zunger SIC applied only to the extra electrons could 
also help.

Some of this is also discussed here, in terms of weakly dipole-bound 
electrons: https://pubs.acs.org/doi/10.1021/acs.jctc.6b00145 .

But in general it's a tough problem, so don't expect a simple solution.

			nicola




On 25/02/2025 02:44, Christoph Wolf wrote:
> Dear all,
> 
> I am facing a tricky system: I am trying to calculate the anion and di- 
> anion of a small polycyclic aromatic hydrocarbon system (~20-25 atoms).
> 
> I am using ONCV PPs and B3LYP just for completeness.
> 
> The neutral molecule is closed shell (66 electrons for example) and the 
> anion (67 e-) is a radical, both converge fast and to meaningful 
> results. However it seems almost impossible to me to stabilize the di- 
> anion (68 e-). A closed shell (total magnetic moment = 0) does not 
> converge at all, a bi-radical (mtot=2) converges numerically but the 
> resulting cell seems to have a lot of charge "outside the molecule" when 
> I calculate the charge difference between anion and di-anion (pp 
> plot_num =0 and then take the difference between two calculations). Also 
> the projwfc.x output for the di-anion has a lot of "nondescript" states:
> 
> 
>   ==== e(  33) =    -4.37227 eV ====
>      |psi|^2 = 0.985
> ==== e(  34) =    -1.39249 eV ====
>      |psi|^2 = 0.001
> ==== e(  35) =    -1.07395 eV ====
> ...
> ==== e(  50) =    -0.36725 eV ====
>      |psi|^2 = 0.933
> 
> 
> Any trick to help the system localize the additional charge better?
> 
> Any help or comment is as always much appreciated!
> 
> Ciao!
> Chris
> -- 
> Christoph Wolf
> IBS Center for Quantum Nanoscience
> Seoul, South Korea
> 
> https://stm25.org/ <https://stm25.org/>
> 
> _______________________________________________________________________________
> The Quantum ESPRESSO Foundation stands in solidarity with all civilians worldwide who are victims of terrorism, military aggression, and indiscriminate warfare.
> --------------------------------------------------------------------------------
> Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
> users mailing list users at lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/users


-- 
----------------------------------------------------------------------
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
Director, National Centre for Competence in Research NCCR MARVEL, SNSF
Laboratory Head, PSI Center for Scientific Computing, Theory, and Data
Contact info and websites at http://theossrv1.epfl.ch/Main/Contact