Mott-Littelton calculation: charge on defect is not zero

GULP
1

Dear Julian,
I am trying to perform a Mott-Littelton calculation using “vacancy” and “interstitial” commands.
I have completely got a result of such commands for the direct substitution mechanism:
2A = B + vacancy.
And it works by this input:

centre 0.62729 0.417555 0.758595
size 10 20
vacancy 0.62823 0.33406 0.75931
vacancy 0.62635 0.50105 0.75788
interstitial B 0.62823 0.33406 0.75931

But if I want to use it in reverse mechanism, such as
B + vacancy = 2A, by input with the same logic:

centre 0.5625 0.20793 0.75
size 10 20
vacancy 0.5 0.25 0.75
#vacancy 0.625 0.16586 0.75
interstitial A 0.625 0.16586 0.75
interstitial A 0.5 0.25 0.75

(note: here I wrote a #vacancy for your undrestanding the position of atom, wich is absent in my new supercell. I mean, this atom
#vacancy 0.625 0.16586 0.75 is a real vacancy in supercell and I want to add it using “interstitial” command),

GULP don’t see the atom on “B” position.
GULP calculate the defect, but the Total number on defect in not zero. It is equal to charge of atom “B”.

I estimated size of radiuses in two regions and I think it is enough.

Are there any problems with syntax and the logic of using “vacancy” and “interstitial” commands?
If no, thank you so much in advance, I’ll try to found a problem with my supercell.

2

Hi Sasha,

I’m not quite sure about what you’re doing with the vacancy in the supercell since this will make things more complicated, especially if it leads to a charged unit cell and so I would just use the M-L method on the regular bulk.
Creating the vacancies and introducing the interstitial should be fine. For your example above you could use the impurity option to replace the atom with B rather than creating a vacancy and then adding the interstitial though the other way is fine. So all looks OK as far as I can tell from this info.