[QE-users] Adsorption energy between charged slab + anion

[Giuseppe Mattioli]

Dear Nam
If you forget the DFT problem of unbound electrons in anions (which is  
not a chemical or physical problem) and think about the chemistry,  
there is no real process that ends with a nitrate anion in gas phase,  
AFAIK. Desorption may rather involve a neutral NO2 molecule, depending  
on the surface redox chemistry. Of course release of anions in  
solution is possible, but you should simulate a (proper or implicit)  
solvation environment.
HTH
Giuseppe

Quoting Stefano Baroni <baroni at sissa.it>:

> If the anion stays charged, which is an issue by itself in DFT, the  
> distance will never be large enough, because the slab will generate  
> a macroscopic electric field. sb
>
> ___
> Stefano Baroni, Trieste -- http://stefano.baroni.me
>
>> On 23 Feb 2021, at 07:05, Nam Tran <vnt981 at uowmail.edu.au> wrote:
>>
>> 
>> Dear QE users,
>>
>> I would like to calculate the adsorption energy of a negative anion  
>> NO3- with a positive slab (The total system is neutral). I cannot  
>> use the convention formula Eds = E(slab+anion) - E(slab) - E(anion)  
>> because of the charge compensation when calculating the energy of  
>> charged slab and charge adsorbate.
>>
>> As the total system (slab+anion) is neutral, I wonder if it is  
>> possible to just move the molecule far away from the slab (make  
>> sure that vacuum in z-direction is large enough), and calculate the  
>> energy difference i.e., one of them the molecule is far away from  
>> the surface and in the other one the molecule adsorbed on the  
>> surface.
>>
>>
>> Best regards
>> Nam
>>
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GIUSEPPE MATTIOLI
CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
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