Dear Bahadir First, charged slabs are problematic because their total energy does not converge with respect to vacuum thickness. You can test on a simple model. However, there is a trick to go around this by inserting a dopant far a way from the critical reaction zone. For example suppose you want to study +1 charged defect on ZrO2 surface. Then you can insert one Y ion(typically 3+ and hence -1 with respect to Zr4+) . This should generate a positive charge somewhere else in the slab and one hopes that this positive charge will localize correctly where you expect it to localize. But one has to be cautious because this also generates a large dipole across the slab. One way to go around this by symmetrizing the slab such that dipoles cancel.
Second, even if charged slabs work, I think you are not conserving the charge when you calculate the binding energy. Because you mentioned you used tot_charge=-1 in the slab+molecule , slab only, molecule only. To me this will not conserve the charge when you calculate the binding energy (B.E.) B.E. =(slab+molec.) - (slab) - (molec) Although I have seen papers defining binding energies that do not conserve the charge, I do not think this is meaningful. Mostafa Y. MIT
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