[QE-users] Negatively charged isolated molecule

[Giuseppe Mattioli]

Dear Ernane
Your question contains part of the answer! Carbonate ion (CO3 2-) is  
not stable outside water, and calculations of its properties in gas  
phase are likely not so meaningful, but in the case of model  
thermodynamics cycles (e.g. Born-Haber). The excess negative charge is  
unbound when not stabilized by a strongly polar solvent, and this is  
likely responsible for instabilities in the construction of the  
Kohn-Sham potential along scf iterations. Moreover, this happens on  
top of the strong delocalization error you experience when you use a  
standard GGA exchange-correlation functional, when the  
self-interaction of strongly localized electrons in the J[n] Coulomb  
potential is not cancelled by a same term in the semi local exchange  
potential. You may minimize this latter source of error by using a  
hybrid GGA-EXX functional such as B3LYP, where the non local   
Hartree-Fock part of the exchange functional can recover part of the  
delocalization error, but you are not free yet from the instability of  
carbonate in gas phase.
HTH
Giuseppe

Ernane de Freitas Martins <ernanefmg at gmail.com> ha scritto:

> Hello,
>
> I'm experiencing a problem to run a negatively charge molecule in quantum
> espresso. The system is CO32-.
>
> I try both vacuum and solvated (environ) calculations. The solvated one
> works fine.
>
> The problem is the calculation in vacuum. It never give the first ionic
> step because the SCF accuracy never reaches the convence criterion.
>
> I tried many different solutions (increase cutoffs and box size, use assume
> isolated, decreasing and changing the mixing scheme and etc) and nothing
> works.
>
> The unique calculation that works fine for vacuum is the one with a box
> size of 7.9 x 7.9 x 7.9 A. I really don't understand why it only works for
> this specific box size.
>
> I ran several other charged systems (+1, +2 and -1 total charge) and all of
> them worked fine. The problem appears for -2 total charge in vacuum.
>
> Would some of you kindly help me in this?
>
> Cheers,
>
> Dr. Ernane de Freitas Martins
> Postdoctoral researcher
> IF - USP
> São Paulo, SP - Brazil



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